Liquid Fertilizer Calculator
Mixing liquid feed by guesswork burns plants. Enter your target nitrogen ppm, fertilizer's N percentage and water volume for the exact grams to add.
Calculate your fertilizer dose
N % is the first number on the bag โ 20-20-20 means 20% nitrogen.
Why ppm, not spoonfuls?
"One tablespoon per gallon" is how plants get burned. A tablespoon of one fertilizer can hold 30% more nitrogen than a tablespoon of another, depending on granule size and density โ and "tablespoon" itself varies by who's scooping. PPM (parts per million) cuts through all of that: it's milligrams of nitrogen per liter of water, an absolute concentration that works for any product.
The formula is straightforward: grams = target ppm ร liters รท (N% ร 10). The ร10 converts the percentage into the ppm scale (1% = 10,000 ppm). So 200 ppm nitrogen in 5 gallons (18.9 L) with 20-20-20 fertilizer: 200 ร 18.9 รท 200 = 18.9 grams, about two-thirds of an ounce. Weigh it on a gram scale and you're exact.
What ppm should you target?
| Plants | Target N ppm | Notes |
|---|---|---|
| Seedlings, young transplants | 50โ100 | Gentle โ roots are small and salt-sensitive |
| Most vegetables & flowers | 100โ200 | The standard weekly feeding range |
| Heavy feeders (tomatoes, roses) | 200โ300 | Peak growth only; back off in heat |
| Houseplants (maintenance) | 75โ150 | Every 2โ4 weeks, not weekly |
When in doubt, start at the low end โ you can always feed again next week, but you can't un-burn roots. Anything sustained over 400 ppm risks leaf-edge burn and salt buildup, especially in containers where salts can't leach away.
Weigh, don't scoop
Liquid feeding uses tiny amounts โ often under 20 grams per batch โ where spoon error dominates. A cheap gram scale (the kind used for coffee or baking) turns guesswork into repeatability: same ppm every time, season after season. Dissolve the weighed fertilizer in a little warm water first, then top up to your full volume and stir.
Signs you've overdone it
- Brown, crispy leaf edges appearing a day or two after feeding โ classic fertilizer burn.
- White crust on the soil surface or pot rim โ salt buildup.
- Wilting despite wet soil โ roots damaged by salts can't take up water.
The fix: flush the pot thoroughly with plain water (several times its volume), skip a feeding, and halve your ppm next round. In-ground beds are more forgiving since rain leaches excess salts away.
Frequently asked questions
How do I calculate fertilizer ppm?
Grams = target ppm ร liters รท (N% ร 10). For 200 ppm N in 5 gallons (18.9 L) with 20% N fertilizer: 200 ร 18.9 รท 200 = 18.9 grams, about 0.67 oz.
What ppm should I feed my plants?
Most vegetables and flowers: 100โ200 ppm. Seedlings: 50โ100 ppm. Heavy feeders in peak growth: 200โ300 ppm. Start low โ you can always feed again.
How do I convert fertilizer percentage to ppm?
A 1% solution = 10,000 ppm. One gram of 20% N fertilizer in 1 liter of water gives 200 ppm nitrogen.
Can I over-fertilize with liquid fertilizer?
Easily. Sustained feeding over 400 ppm nitrogen risks leaf burn and salt buildup, especially in containers. Brown leaf edges after feeding mean: flush with water, halve the rate.
Should I use a scale or measuring spoons for fertilizer?
A gram scale โ a "teaspoon" can vary 30% by packing density. For the small doses liquid feeding uses, weighing is the only way to hit your ppm reliably.