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About this tool
Size a dehumidifier in litres per day from room volume, damp severity and humidity, with runtime and running cost.
A dehumidifier capacity calculator converts the litres of water a room gains each day into the nameplate extraction rate you need to buy. It builds a moisture balance from room volume, air changes per hour, and the humidity ratio difference between your current and target relative humidity — using the Magnus saturation-vapour equation rather than a generic square-footage chart — then adds the water people, laundry and showers release. Because catalogue litres are measured at 30 °C and 80% RH, the result is also derated to the conditions the machine will really see.
Open Dehumidifier Capacity Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Applies the derating that turns 20 rated litres into roughly 11 real ones in cooler, drier air.
Ranks infiltration, occupants, laundry and wall evaporation so you can fix the cause, not just the symptom.
Reports duty cycle, kWh per month and rupees per month at your own tariff before you buy.
For a 400 sq ft room with a 10 ft ceiling sitting at 30 °C and 80% RH with two occupants, the moisture load works out to roughly 18–19 litres a day, which needs a unit rated about 30 L/day. The rated figure is much larger than the load because nameplate litres are measured at 30 °C / 80% RH and real extraction drops as the air dries out.
Because the rating is measured at 30 °C and 80% RH, and extraction falls roughly in proportion to the humidity ratio of the air being fed to it. At 25 °C and 60% RH a unit typically delivers around half its rated litres; US AHAM ratings taken at 18.3 °C and 60% RH read lower again for the identical machine.
Between 45% and 55% RH suits most homes: it is dry enough to stop dust mites and mould, which need sustained humidity above about 60%, without drying out timber, instruments or your sinuses. Every extra 5% you dial down raises both runtime and the electricity bill, so do not chase 35% unless you are drying out a flood.
A compressor dehumidifier removes roughly 2 litres per kWh at its rating point and less in drier air, so a room shedding 18 litres a day can consume 10–12 kWh a day while it runs. Sealing draughts, venting the bathroom and drying laundry outdoors cut the load directly and are usually cheaper than a bigger machine. For persistent structural damp, consult a surveyor before buying any appliance.
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