Fertiliser Dilution Calculator
Convert a dilution ratio into an amount per watering can.
Read the label first, then use this
Product concentrations vary enormously between brands and between formulations of the same brand, and the label instruction is specific to what is in that bottle. This calculator converts a ratio you have already read — 1:100, or 5ml per litre — into the amount for your particular watering can. It cannot tell you what the correct ratio is, and guessing at it is the main way plants get burned.
Where two instructions conflict, the more dilute one is the safer default. Half-strength feed applied twice as often is gentler on roots than full strength applied occasionally.
What N-P-K actually tells you
The three numbers are percentages by weight of nitrogen, phosphorus (as P₂O₅), and potassium (as K₂O). Nitrogen drives leafy growth, phosphorus supports roots and establishment, potassium supports flowering and fruiting and general hardiness. A high-nitrogen feed on tomatoes produces impressive foliage and few fruit, which is why tomato feeds are potassium-heavy. The numbers also indicate strength: a 20-20-20 product is roughly twice as concentrated as 10-10-10 and needs proportionally more dilution.
Over-feeding is more common than under-feeding
Excess soluble fertiliser draws water out of roots by osmosis, producing scorched leaf margins, wilting despite moist soil, and a white salt crust on the soil surface. Container plants are most at risk because salts accumulate with nowhere to leach; flushing pots thoroughly with plain water every few weeks clears the build-up. Feeding a plant that is stressed, dormant, or already wilting makes matters worse rather than better — water first, feed later, and never feed dry soil.
Runoff matters beyond the garden. Nutrients washed from over-fertilised soil into waterways drive algal blooms, which is a good practical reason to apply the amount the plant will use rather than the amount the bottle would like you to buy.
How to use it
- Read the dilution ratio from your product label.
- Enter the ratio and the capacity of your watering can.
- Water the plant first, then apply the feed to already-moist soil.
- Flush containers with plain water every few weeks to clear salt build-up.
Before you plant
What does a 1:100 dilution mean?
One part concentrate to 100 parts water — 10ml in a 1-litre can, or 100ml in a 10-litre can. Some labels instead state millilitres per litre directly, which is the same idea expressed differently.
What do the N-P-K numbers mean?
They are percentages of nitrogen, phosphorus, and potassium by weight. Nitrogen drives leafy growth, phosphorus supports roots, and potassium supports flowering and fruiting. They also indicate concentration — 20-20-20 is about twice as strong as 10-10-10.
Can I use a stronger mix for faster growth?
No. Excess soluble fertiliser pulls water out of roots and causes scorched leaf edges and wilting in moist soil. Half strength applied more often is safer than full strength applied occasionally.
Should I feed dry soil?
Never. Water thoroughly first and apply feed to moist soil, otherwise the concentrated solution contacts dry roots directly and burns them.
Disclaimer: These are general planning estimates. Seed packets, product labels, and regional frost, water, and pesticide guidance always take priority over any figure produced here. Local soil, climate, and variety differences can change the result substantially.