Fertiliser Dilution Calculator

This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.

Liquid plant feed is sold as a concentrate, and the label gives a dose like 5 millilitres per litre or one teaspoon per gallon. This calculator turns that into the exact amount for your own watering can. It also lets you feed at half or quarter strength, which many houseplants prefer, since they grow more slowly than the outdoor plants most labels are written for. Feed only when a plant is in active growth, and go easy in winter for most plants.

How this works

You enter the dose from your label, the size of your watering can, and the strength you want to feed at. I convert the dose to a per-litre amount, scale it to your can size, and apply the strength you chose. The result is how much feed to add and how much water to fill up with. I also give teaspoons, since most home feeding is measured with a spoon.

Understanding your result

The feed figure is how much concentrate to pour in. Fill the rest of the can with water up to the size you entered. Extension services often suggest feeding houseplants at half or even quarter of the label strength, and a little more often, since indoor plants grow slower and the label rates are written for fast-growing outdoor plants. Too much feed builds up as salt in the soil and shows as brown leaf tips.

Important limitations

This does the maths from the dose you give it. It cannot know your plant, your light, or the season, which all change how much feed a plant wants. Feed only during active growth, usually spring and summer, and go easy in low light. If you see a white crust on the soil or brown leaf tips, cut back or flush the pot with plain water.

Scientific basis

The dilution and half-strength guidance behind this tool comes from university extension services, the public gardening advice arm of the land-grant university system:

• Iowa State University Extension — How Often Should I Fertilize Houseplants
• N.C. Cooperative Extension — How to Fertilize House Plants
• University of Maryland Extension — Fertilizer for Indoor Plants

Related reading

My guide on why houseplant leaves turn yellow covers feeding mistakes and salt buildup, and my article on how often to water houseplants pairs well with a feeding routine. My NPK calculator helps you read the three numbers on a fertiliser label.

This calculator is for educational purposes and general guidance only. It is not professional horticultural advice. Always read and follow your fertiliser product label.