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Masterblend 4-18-38 is the cult-favorite hydroponic nutrient mix for tomatoes, peppers, and leafy greens — and at roughly 8 cents per gallon it’s also the cheapest credible hydroponic nutrient line on the market. The catch is that the recipe is published in increments of 5 gallons (12g Masterblend, 12g Calcium Nitrate, 6g Epsom Salt) and most home growers run reservoir sizes that aren’t 5 gallons. A 4-liter AeroGarden Bounty needs different gram measurements. So does a 20-liter NFT system, a 1-gallon Kratky bucket, or a 100-gallon greenhouse trough.
The math is linear and easy to do by hand, but it’s the kind of math that’s annoying to do by hand every time you mix a fresh reservoir. So we built a calculator. Enter your reservoir size, choose the tomato formula or the lettuce formula, and the tool returns the exact gram measurements for all three components.
The calculator is below. The reference tables, mixing instructions, and the explanation of why you need to mix the components separately are below that.
The Calculator
Reference Recipe Tables
If you’d rather look up your reservoir size in a table than use the calculator (or you’re on a device without JavaScript), here are the most common sizes pre-calculated.
Tomato / Vegetable Formula — Masterblend 4-18-38
The all-purpose recipe. Works for tomatoes, peppers, cucumbers, strawberries, basil, and most leafy greens. This is the formula Masterblend International publishes as the standard 4-18-38 Tomato Formula and the one most DIY hydroponic growers use as their default.
| Reservoir size | Masterblend 4-18-38 | Calcium Nitrate (15.5-0-0) | Epsom Salt |
|---|---|---|---|
| 1 US gallon (3.79 L) | 2.4 g | 2.4 g | 1.2 g |
| 2 US gallons (7.57 L) | 4.8 g | 4.8 g | 2.4 g |
| 3 US gallons (11.36 L) | 7.2 g | 7.2 g | 3.6 g |
| 5 US gallons (18.93 L) | 12.0 g | 12.0 g | 6.0 g |
| 10 US gallons (37.85 L) | 24.0 g | 24.0 g | 12.0 g |
| 20 US gallons (75.7 L) | 48.0 g | 48.0 g | 24.0 g |
| 50 US gallons (189 L) | 120.0 g | 120.0 g | 60.0 g |
| 100 US gallons (379 L) | 240.0 g | 240.0 g | 120.0 g |
| ½ liter (~0.13 gal) | 0.32 g | 0.32 g | 0.16 g |
| 1 liter | 0.63 g | 0.63 g | 0.32 g |
| 4 liters (AeroGarden Bounty) | 2.54 g | 2.54 g | 1.27 g |
| 7.5 liters (LetPot LPH-Max) | 4.76 g | 4.76 g | 2.38 g |
| 20 liters | 12.69 g | 12.69 g | 6.34 g |
Lettuce Formula — Masterblend 8-15-36
A separate Masterblend formula optimized for leafy greens. Lower phosphorus, higher potassium, slightly different ratios. Use this if you’re growing exclusively lettuce, spinach, kale, arugula, or other leafy greens — it produces faster vegetative growth without the bloom-bias of the 4-18-38.
| Reservoir size | Masterblend 8-15-36 | Calcium Nitrate (15.5-0-0) | Epsom Salt |
|---|---|---|---|
| 1 US gallon | 2.2 g | 1.7 g | 1.3 g |
| 2 US gallons | 4.4 g | 3.4 g | 2.6 g |
| 3 US gallons | 6.6 g | 5.1 g | 3.9 g |
| 5 US gallons (seedling/young plants) | 11.0 g | 8.5 g | 6.5 g |
| 10 US gallons | 22.0 g | 17.0 g | 13.0 g |
| 20 US gallons | 44.0 g | 34.0 g | 26.0 g |
| 50 US gallons | 110.0 g | 85.0 g | 65.0 g |
| 100 US gallons | 220.0 g | 170.0 g | 130.0 g |
| 1 liter | 0.58 g | 0.45 g | 0.34 g |
| 4 liters (AeroGarden Bounty) | 2.33 g | 1.80 g | 1.37 g |
| 7.5 liters (LetPot LPH-Max) | 4.36 g | 3.37 g | 2.58 g |
| 20 liters | 11.63 g | 8.99 g | 6.87 g |
Note on the lettuce formula: Masterblend’s published lettuce recipe (11g / 8.5g / 6.5g per 5 gallons) is for seedlings and young plants. For mature lettuce in production growth you can run slightly higher concentrations — roughly 14g / 11g / 8g per 5 gallons. We default the calculator to the seedling concentration because it’s more forgiving for new growers.
Mixing Instructions (Critical — Read This Before You Mix Anything)
The single most common mistake new Masterblend users make is combining all three dry components together before adding them to water. Do not do this. It causes immediate nutrient lockout — the calcium from the Calcium Nitrate reacts with the phosphates and sulfates in the Masterblend and Epsom Salt to form insoluble calcium phosphate and calcium sulfate precipitates that drop out of solution and never become available to your plants.
The correct procedure is to dissolve each component separately in the full reservoir volume of water, in the right order, before adding the next one. Here’s the step-by-step.
Step 1 — Start with the right water
Use distilled water, reverse osmosis (RO) filtered water, or low-mineral tap water. Don’t use tap water from a hard-water area — the dissolved calcium and magnesium in hard tap water complicate the recipe and can produce the same precipitate problem the mixing order is designed to prevent. If you’re not sure about your tap water hardness, use distilled water as the default. A gallon of distilled water at any US grocery store costs about $1.50 and a 5-gallon mix uses 5 gallons of distilled water (about $7.50 of water — not free, but not significant relative to the cost of the system).
Fill your reservoir to its target volume with water before adding any nutrients.
Step 2 — Add the Masterblend powder (Component A)
Measure out the Masterblend 4-18-38 (or 8-15-36 for lettuce) using a digital scale accurate to 0.1 grams. Cheap kitchen scales work fine — you don’t need a lab scale.
Sprinkle the Masterblend powder slowly into the reservoir while stirring gently with a clean utensil. Continue stirring until the powder is fully dissolved. Wait about 60 seconds for the solution to equilibrate before moving to the next component.
Step 3 — Add the Epsom Salt (Magnesium Sulfate)
Measure and add the Epsom Salt the same way. Stir until fully dissolved. Wait another 60 seconds.
You can mix Masterblend and Epsom Salt together in the same step (some published recipes do this) — they don’t react badly with each other. The hard rule is that the Calcium Nitrate has to go in last and separately.
Step 4 — Add the Calcium Nitrate (Component B) LAST
Measure and add the Calcium Nitrate. Stir until fully dissolved. This must be the last component you add. Calcium Nitrate added to the reservoir after the Masterblend and Epsom Salt is fully dissolved will not produce precipitates because the calcium ions distribute through the already-equilibrated solution rather than encountering concentrated phosphate and sulfate clumps.
After all three components are dissolved, the solution should be clear (or only very slightly tinted). If it’s cloudy or has visible white precipitates floating in it, something went wrong with the mixing order — drain the reservoir and start over.
Step 5 — Test pH
The fresh Masterblend recipe with distilled water typically lands at pH 5.8–6.2, which is in the right range for most edibles. Test with a pH meter to confirm. If you’re outside the 5.5–6.5 window, adjust with food-grade pH-down (phosphoric acid) one drop at a time until you’re in range.
We have a full guide on pH management for AeroGarden and small-reservoir hydroponics that goes deep on this — it’s the single most common preventable problem in home hydroponic growing.
What You Need to Buy
The full Masterblend kit is sold as a “combo” package on Amazon and direct from PowerGrow Systems (the most common Masterblend distributor). The combo includes all three components in a single box, which is cheaper and more convenient than buying them separately.
| Item | Where to buy | Approx price |
|---|---|---|
| Masterblend 4-18-38 + Calcium Nitrate + Epsom Salt combo (2.5 lb starter kit) | Amazon | ~$30 |
| Masterblend 4-18-38 + Calcium Nitrate + Epsom Salt combo (5 lb) | Amazon | ~$50 |
| Masterblend 4-18-38 + CalNit + Epsom (25 lb bulk) | PowerGrow direct | ~$150 |
| Lettuce Formula Masterblend 8-15-36 (sold separately, less common) | PowerGrow direct | ~$35 |
| Digital kitchen scale (0.1g accuracy) | Amazon | ~$15 |
| Distilled water (1 gallon) | Any US grocery store | ~$1.50 |
| Hydroponic pH-down (food-grade phosphoric acid) | Amazon | ~$10 |
| HM Digital COM-80 EC/TDS pen (full review) | Amazon | ~$35 |
Total starter cost: about $90 for the full kit, scale, water, pH-down, and the entry-tier meter.
Compared to buying pre-mixed liquid hydroponic nutrients — General Hydroponics Flora Series 3-part trio runs about $45 for 1-quart bottles that last a few weeks of feeding — the Masterblend dry mix pays for itself in roughly the second month. By month 6, the per-gallon cost is roughly 10x cheaper than any pre-mixed liquid line. This is why Masterblend is the cult-favorite among serious DIY hydroponic growers, especially Kratky-method users running multiple reservoirs.
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Why Masterblend Instead of Pre-Mixed Liquid Nutrients
This section is for readers wondering whether the extra mixing complexity is worth the cost savings. Short answer: for anyone running more than one hydroponic reservoir or mixing fresh nutrient solution more than once a month, Masterblend is the better economic choice. For a single AeroGarden Bounty owner mixing one reservoir every 2-3 weeks, pre-mixed liquid nutrients are simpler and the cost savings don’t justify the mixing learning curve.
The honest tradeoffs:
Masterblend wins on:
- Cost per gallon — roughly $0.08 per gallon of mixed nutrient solution at the 5-pound combo size, or ~$0.03 per gallon at the 25-pound bulk size. By comparison, General Hydroponics Flora Series runs $0.35-$0.50 per gallon of equivalent strength solution, and Click & Grow’s proprietary nutrients work out to around $1.20 per gallon when amortized across the pod subscription cost.
- Shelf life — dry Masterblend stored in a sealed container is stable for years. Liquid nutrients have a shelf life of typically 1-2 years and can settle, separate, or grow microbial contamination if stored too long.
- Storage volume — 5 pounds of Masterblend dry mix produces roughly 100-150 gallons of nutrient solution. The same volume of pre-mixed liquid would take a LOT of shelf space.
- Recipe customization — you can adjust the ratios for specific crops (tomato vs lettuce vs strawberry) without buying separate product lines.
Pre-mixed liquid wins on:
- Simplicity — pour, dose, done. No scale, no mixing order, no precipitate risk.
- Lower learning curve — for first-time hydroponic growers, the mixing complexity of Masterblend is a real barrier.
- No scale required — liquid nutrients are dosed in milliliters with included measuring caps. No equipment beyond what’s in the box.
- Less room for error — mess up the Masterblend mixing order and you waste a reservoir. Mess up the liquid nutrient dose and you over-fertilize but the nutrients stay soluble.
The honest recommendation: if you own a single AeroGarden and you’re growing herbs and lettuce, General Hydroponics MaxiGro is the right buy at $22 — it’s a one-bag dry powder that doesn’t have the mixing-order problem and is forgiving for beginners. If you own multiple hydroponic systems, run Kratky buckets, grow tomatoes or peppers in volume, or want the cheapest possible per-gallon cost, Masterblend is the right buy. The calculator above and the recipe tables make the mixing math trivial; the only learning curve is the dissolve-in-order procedure.
Frequently Asked Questions
Why do you have to mix the three components separately?
Calcium ions (from Calcium Nitrate) react with phosphate ions (from the Masterblend’s potassium phosphate component) and sulfate ions (from both Masterblend and Epsom Salt) to form insoluble calcium phosphate and calcium sulfate precipitates. These precipitates drop out of solution and become unavailable to your plants. The mixing-order procedure (Masterblend first, Epsom Salt second, Calcium Nitrate last) prevents the calcium from encountering high local concentrations of phosphate and sulfate, which keeps everything in solution.
If you accidentally combine the dry components before adding to water, you’ll see visible white precipitate when you finally do add it to water. Drain the reservoir and start over.
Can I use the tomato formula for lettuce?
Yes, and most growers do. The 4-18-38 tomato formula works fine for lettuce, herbs, leafy greens, and basically any edible crop. The lettuce-specific 8-15-36 formula produces slightly faster vegetative growth in pure leafy-green setups but the difference is small. If you’re only buying one Masterblend product, buy the 4-18-38 tomato formula — it’s more universally available, more universally compatible, and works for everything.
Can I use the lettuce formula for tomatoes?
Not optimally. The 8-15-36 lettuce formula has lower phosphorus than tomatoes need during fruiting, and the ratios are biased toward vegetative growth rather than flower set and fruit development. Use the 4-18-38 tomato formula for tomatoes, peppers, cucumbers, and strawberries.
How often do I need to mix a fresh batch?
Depends on reservoir size and how aggressively your plants are feeding. Rough guideline:
- Small reservoirs (AeroGarden, LetPot, small Kratky): every 2-3 weeks, or sooner if pH or EC drift outside the target range
- Medium reservoirs (5-10 gallon DWC, vertical towers): every 3-4 weeks
- Large reservoirs (20+ gallon NFT, greenhouse): every 4-6 weeks
The maintenance cycle is dictated by pH/EC drift, not by a fixed schedule. Test weekly and refresh when readings move outside the safe window. We have a companion guide on pH management that explains why drift happens and how fast.
Can I scale the recipe for very small reservoirs (under 1 gallon)?
Yes, but accuracy matters more at small scales. For a half-gallon reservoir using the tomato formula, you need 1.2 g Masterblend, 1.2 g Calcium Nitrate, and 0.6 g Epsom Salt. That’s small enough that an inaccurate kitchen scale (one that’s only accurate to 1 gram) will produce meaningfully wrong concentrations. Use a scale accurate to at least 0.1 grams for any reservoir under 5 gallons. Cheap 0.1-gram scales sell for around $15 on Amazon and are good enough.
What’s the difference between Calcium Nitrate (15.5-0-0) and Calcium Nitrate (Yara Calcinit, etc.)?
Slightly different formulations from different manufacturers. The 15.5-0-0 nitrogen analysis is the most common spec and the one most Masterblend recipes are calibrated for. Yara’s Calcinit (the most common European calcium nitrate) is also 15.5-0-0 and is interchangeable with the US-sold 15.5-0-0 product. As long as the nitrogen analysis is 15.5-0-0 or very close to it, the recipe works as-is.
Can I store mixed nutrient solution for later?
Not for very long. Mixed Masterblend solution is stable for about 1-2 weeks if kept in a sealed, light-blocking container at cool room temperature. After that, microbial growth and pH drift can affect quality. The right move is to mix fresh batches as needed, not to mix large batches and store them. If you’re worried about waste, mix smaller batches more often.
The dry components have effectively unlimited shelf life if kept dry and sealed.
Is Masterblend safe to handle?
Yes, with normal precautions. The components are agricultural fertilizers, not household chemicals — but they’re concentrated, so wash your hands after handling, don’t get the powder in your eyes, and keep the dry containers away from children and pets. The mixed solution at recommended dilution is roughly equivalent in skin-contact safety to ordinary garden fertilizer.
Can I use Epsom Salt from the pharmacy instead of food-grade?
Yes. Pharmacy-grade Epsom Salt and food-grade Epsom Salt are both magnesium sulfate. The pharmacy version is typically labeled for bath use and is identical chemically. Don’t use any “Epsom Salt” that’s been scented or had additives mixed in — pure magnesium sulfate only.
What about Masterblend in Australia or Europe?
Masterblend has limited availability outside the US — the brand and the 4-18-38 formula are American products and shipping costs from US distributors to AU/EU make it impractical for most growers in those markets. For Australian and European hydroponic growers, the best alternatives are:
- Australia: Manutec Hydroponic Nutrient or Nutrifield ProSeries — both are locally made dry hydroponic nutrient kits with similar economics to Masterblend
- Europe: General Hydroponics Flora Series (widely distributed in EU), Canna Aqua Vega + Flores (NL-made, very widely available), or Hesi (NL) — these are liquid nutrients rather than dry powders, but they’re the closest practical equivalent for the EU market We have a full hydroponic nutrients buyer’s guide that covers the regional availability question in more detail.
Is the calculator math actually correct?
Yes, and you can verify it by hand against the Masterblend International published 5-gallon recipes. Per-gallon ratios:
- Tomato 4-18-38: 12g / 12g / 6g per 5 gallons → 2.4 g / 2.4 g / 1.2 g per gallon
- Lettuce 8-15-36: 11g / 8.5g / 6.5g per 5 gallons → 2.2 g / 1.7 g / 1.3 g per gallon
The calculator multiplies the per-gallon ratio by your input volume. For liter inputs, it converts to gallons first using the standard 3.78541 L/gal conversion. The math is intentionally simple and linear — no surprise rounding, no temperature corrections, no buffering coefficients. If you find a discrepancy between the calculator output and the published recipes, please report it and we’ll fix it.
Bottom Line
Masterblend 4-18-38 is the cheapest credible hydroponic nutrient line on the market and the cult-favorite for serious DIY food growers. The recipe scales linearly to any reservoir size — use the calculator above for one-off mixing, or the reference tables for the most common sizes. The only thing you absolutely have to get right is the mixing order: dissolve the Masterblend first, the Epsom Salt second, and the Calcium Nitrate LAST and SEPARATELY into the already-mixed solution. Combining all three dry components before adding to water causes immediate nutrient lockout and ruins the batch.
For most home growers, the right starter buy is:
- Masterblend 4-18-38 + Calcium Nitrate + Epsom Salt combo, 5-pound size — about $50 (buy on Amazon)
- Digital kitchen scale, 0.1g accuracy — about $15 (buy on Amazon)
- Distilled water from the grocery store
- HM Digital COM-80 EC/TDS pen for monitoring — about $35 (full review)
Total starter cost: roughly $100 including the meter. After that, the per-gallon cost of mixed nutrient solution is around $0.08 — about 10x cheaper than pre-mixed liquid alternatives like General Hydroponics Flora Series.
If you’re a single AeroGarden Bounty owner growing only herbs and lettuce, Masterblend is overkill — buy the General Hydroponics MaxiGro one-bag dry powder instead and save yourself the mixing complexity. If you’re running multiple hydroponic systems, growing tomatoes or peppers in volume, or you want the cheapest possible per-gallon cost over the long term, Masterblend is the right answer.
Methodology note. Recipe ratios in this calculator and the reference tables are derived from the published Masterblend International 4-18-38 Tomato Formula and 8-15-36 Lettuce Formula 5-gallon recipes. The calculator math is intentionally linear and is verified against the published recipes at the 5-gallon reference point. Per-gallon ratios are arithmetic conversions of the published 5-gallon recipes. We have not modified the Masterblend recipes — only made them easier to scale. Read our full testing methodology.
Last verified pricing: 2026-04-08. Report a stale price.
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