Aquarium dosing calculator
Aquarium products give their dose for a set amount of water, and your tank is almost never that amount. Enter the dose from the label and your water volume, and the calculator scales it for you, in millilitres, drops or teaspoons.
Dose on the label
Copy the dose from the bottle or packet, for example 5 ml per 40 litres.
Your tank
Use the water actually in the tank, not the size on the box. Work out your real water volume.
Enter the label dose and your water volume to see your dose.
How the dose is worked out
The label dose is divided by the volume it treats, then multiplied by your volume. A product dosed at 5 ml per 40 litres works out at 0.125 ml per litre, so a 150-litre tank needs 18.75 ml. The label and your tank can use different units: the calculator converts between litres, US gallons and UK gallons.
The answer stays in the unit the label uses. Liquid doses are also shown in the other common measures, so you can dose with whatever you have to hand. Powder doses in grams are not converted to spoons, because how much a spoonful weighs depends on the powder.
Dose for the water, not the box
Most tanks hold noticeably less water than the size printed on them, once the glass, the substrate and the gap below the rim are taken off. A 60 × 30 × 36 cm tank sold as 65 litres can hold about 46 litres in use, so dosing for 65 would give about 40% more than intended. That matters most with medication, where the gap between a dose that works and one that harms can be small. The volume calculator works out the water your tank really holds.
For a water change, check the label. Water conditioner is often dosed for just the new water going in, but some labels say to dose for the whole tank, and most medications and supplements are dosed for the whole tank.
Measuring small doses
- An oral syringe marked in 0.1 ml steps is the most accurate way to measure a liquid dose, and it costs very little.
- Caps and built-in measures are fine for large doses but hard to read below a few millilitres.
- Drops vary with the dropper and the liquid. The calculator counts 20 drops to the millilitre, the usual pharmacy figure, so treat any drop count as a guide.
- Weigh powders on a scale that reads to 0.01 g rather than measuring them with spoons.
Before you dose
- Read the whole label. Some products are dosed once, some daily and some over a course of several days.
- If the label says to, remove carbon and other chemical filtration while treating with medication, because it can take the medicine out of the water.
- Add the dose where the flow is strongest, so it mixes quickly.
- Many medications are harmful to shrimp, snails and corals. Check the label before treating a tank that has them, and watch your livestock after dosing.
Common questions
How do I work out the dose for my tank size?
Divide the dose by the volume on the label, then multiply by your tank's volume. For 5 ml per 40 litres in a 150-litre tank: 5 ÷ 40 × 150 = 18.75 ml.
How many drops are in 1 ml?
About 20, the usual pharmacy figure. Aquarium droppers vary with the bottle and the liquid, so the real number can be noticeably different. Where accuracy matters, measure in millilitres.
Should I dose for the tank size or the water volume?
The water volume. The size on the box usually describes the outside of the tank filled to the brim, which can be 20 to 30% more than the water it really holds.
What is the difference between US and UK gallons?
A US gallon is 3.785 litres and a UK gallon is 4.546 litres. American products usually mean US gallons, so 1 ml per 10 gallons on a US product means 1 ml per 37.85 litres.
Can I use a teaspoon instead of millilitres?
A measuring teaspoon holds 5 ml, so it works for larger liquid doses. Kitchen teaspoons vary in size, so use a proper measuring spoon, and weigh powders rather than spooning them.
