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Dehumidifier Size Calculator: What Capacity Do I Need?

Enter the floor area of the space, describe how damp it is and count any extra moisture sources. The calculator uses the classic industry sizing chart to give the daily water removal you need, then suggests a unit size to shop for.

m²
Count 1 each for: humid climate, washer/dryer in the space, several occupants, many doors or windows.

Includes the waste margin explained below. Check your product label for coverage and sizes.

How the dehumidifier size calculator works

Dehumidifiers are sold by how much water they pull out of the air in 24 hours. The amount a space needs depends on two things: how big it is and how much moisture keeps coming in through walls, floor, people and appliances. The calculator follows the long-standing AHAM sizing chart, which gives a base figure for each dampness level and adds a little more for every extra 500 ft² (46 m²) of floor.

Pints/day = (5 + k) + (0.006 + 0.002 k) × area in ft² + 5 × extra factors

k is the dampness level from 1 (moderately damp) to 4 (extremely wet). Each extra factor, such as a tumble dryer in the room or a humid summer climate, adds 5 pints (2.4 L) a day. One US pint is 0.473 L. Because the chart was written for the older rating test at 80 °F (26.7 °C), the calculator also shows the equivalent on the 2019 US Department of Energy test at 65 °F, which gives figures roughly 30% lower for the same machine. When the need is larger than the biggest common portable unit (60 pints on the 2019 label, 50 L/day), it shows how many of those units it takes: an extremely wet 10,000 ft² space needs 104 pints a day, so two 60-pint units.

Worked example

A 60 m² basement is about 646 ft². It always smells musty (level 2) and has a washing machine, so one extra factor. The need is 7 + 0.010 × 646 + 5 = 18.5 pints a day, which is 8.7 L per day. On a European shelf a 10 L/day unit covers it; in the US the same need is about 13 pints on the current label, so the smallest common size of 22 pints (2019 rating) is ample. A 650 ft² space in the same state gives 18.5 old-style pints, or 13 on the new rating.

Sizing chart (pints per day, original AHAM basis)

Condition500 ft² (46 m²)1,000 ft² (93 m²)1,500 ft² (139 m²)2,000 ft² (186 m²)
Moderately damp10141822
Very damp12172227
Wet14202632
Extremely wet16233037

Multiply by 0.47 for litres per day. Old 70-pint units are now labelled around 50 pints, and old 50-pint units around 35, even though nothing about the machine changed.

Reading the label correctly

  • European ratings are often quoted at 30 °C and 80% relative humidity, a hot and very humid test. In a cool cellar at 15 °C the same unit removes far less. Many spec sheets also list capacity at 27 °C / 60%, which is the figure to compare with this calculator.
  • Cold rooms favour desiccant models. Compressor units lose most of their capacity below about 15 °C and can ice up; desiccant units keep working in unheated garages and boats.
  • Target 45–55% relative humidity. Below 60% mould and dust mites struggle; below 40% the air feels dry. A cheap hygrometer tells you whether the unit is keeping up.
  • Drainage matters as much as size. A 10 L tank that fills in a day needs emptying daily; a continuous hose to a floor drain or a built-in pump saves the chore.
  • Fix the source first. Blocked gutters, a missing damp-proof course or a dryer venting indoors will outrun any dehumidifier. Persistent water ingress calls for a damp specialist.

Frequently asked questions

What size dehumidifier do I need for a basement?

A 1,000 ft² (93 m²) basement that always smells musty needs about 17 pints (8 L) per day on the original chart, which is a 22-pint unit on today's US label or a 10–12 L unit in Europe.

What size dehumidifier for a 1,500 sq ft space?

From 18 pints a day if moderately damp up to 30 pints if extremely wet, on the original rating. On the 2019 label that is roughly 13 to 21 pints, so a 22 or 35 pint unit.

Is a 50 pint dehumidifier the same as a 70 pint?

Usually yes. Since 2019 US units are tested at 65 °F instead of 80 °F, so a machine once sold as 70 pints is now rated about 50 pints.

How many litres a day should a dehumidifier remove?

For a bedroom or small flat with condensation, 10–12 L per day (at 30 °C / 80%) is typical; a damp house or laundry drying indoors calls for 16–20 L.

Can a dehumidifier be too big?

An oversized unit is not harmful; it simply cycles off sooner once it reaches the set humidity, but it costs more to buy and is often louder.

Last reviewed: 2026-09-27 · How we calculate