Fertilizer Ratio Converter

Fertilizer labels show an NPK ratio, three numbers representing the percentage of:

• N = Nitrogen (leaf and stem growth)
• P = Phosphate, written P₂O₅ (root and flower development)
• K = Potash, written K₂O (overall plant health)

How to read NPK: A bag labeled 10-10-10 contains:

• 10% nitrogen, 10% phosphate, 10% potash
• The rest (70%) is filler/carrier material

Only the first number is an element. This catches out almost everyone the first time. The nitrogen figure really is elemental nitrogen, but the second and third are quoted as oxides, a convention inherited from nineteenth-century soil chemistry that nobody has managed to retire. To get the actual elements:

• Elemental phosphorus = P₂O₅ × 0.436 (because P is 61.95 of the 141.94 g/mol in P₂O₅)
• Elemental potassium = K₂O × 0.830 (because K is 78.20 of the 94.20 g/mol in K₂O)

So a 50-pound bag of 10-10-10 holds 5 lb of nitrogen, 5 lb of phosphate containing 2.2 lb of actual phosphorus, and 5 lb of potash containing 4.2 lb of actual potassium. Soil test reports often give elemental figures while the bag gives oxides, and comparing the two directly overstates your phosphorus by a factor of 2.3. The calculator above prints both.

Application calculation:

• Nutrient per bag = Bag weight × (% / 100)
• Example: 50 lb bag of 10-10-10 contains 5 lb of each nutrient

Common NPK ratios:

Liquid fertilizer dilution:

• Rate is usually given as tablespoons per gallon or mL per liter
• 1 tablespoon = 14.79 mL
• 1 US gallon = 3.785 liters

There’s a difference between a fertilizer’s ratio and its strength that trips up gardeners. A 10-10-10 and a 20-20-20 share the same 1-1-1 ratio, but the second is twice as concentrated, so you use half as much for the same feeding. A 50-pound bag of 10-10-10 delivers five pounds each of nitrogen, phosphate and potash, with the remaining 35 pounds as carrier.

Application rates: the number that matters is nitrogen. Lawn feeding is specified as pounds of nitrogen per 1,000 square feet, not pounds of product, and the standard rate is 1 lb N per 1,000 sq ft per feeding. Rearranged:

Product needed (lb) = (target lb N per 1,000 sq ft × area in sq ft / 1,000) × 100 / N%

At the standard rate, a 20-0-10 lawn feed needs 5 lb of product per 1,000 sq ft, while a 10-10-10 needs 10 lb to deliver the same nitrogen. That is the whole reason the concentration matters, and it is why swapping products without redoing the arithmetic is the usual cause of a burnt lawn. An acre is 43,560 square feet, so multiply the per-1,000 figure by 43.56 for acre rates.

A couple of things the label won’t tell you. Most established soils already hold plenty of phosphorus, so the high-middle-number “bloom booster” products are oversold for a lot of gardens; a soil test is the only way to know what you’re actually short on. And nitrogen is the one to respect, because it’s the number that burns. Too much, or a fast-release form on a dry lawn, scorches the grass in days. Slow-release nitrogen feeds gently over weeks and is far more forgiving, usually the better choice for anyone not measuring carefully.