Dehumidifier & ACH Room Sizer
Turn room size and dampness into a dehumidifier size
Dehumidifier sizing, ACH and humidity FAQ
What indoor humidity should I aim for?
Most health and building guides suggest keeping indoor relative humidity roughly 30–50% RH in normal living spaces. Above about 60% RH, mold and dust mites are more likely to thrive, and you may see foggy windows, musty smells and peeling paint. A small digital hygrometer is the easiest way to see where your room sits.
What does “ACH” mean on this page?
ACH stands for air changes per hour – how many times the total room air volume is effectively exchanged or mixed in an hour. For moisture control in homes, many guides sit around 0.5–1.5 ACH depending on how damp the space is. This calculator turns your room volume into a simple ACH band and a corresponding airflow suggestion.
How accurate is the pint/day dehumidifier size?
The number here is a rule-of-thumb starting point. It uses floor area and dampness level, with a safety buffer so you are more likely to be slightly oversized than undersized. Manufacturer charts, ENERGY STAR guidance and retailer recommendations will often give a very similar range, but you should still compare with real product tables.
Why does the tool ask for ceiling height?
Many sizing charts assume an 8 ft (2.4 m) ceiling. Taller or lower rooms change the actual air volume, which affects both moisture load and ACH. By including height, the calculator can give a better match for attics, basements and high-ceiling rooms.
Do I really need to enter humidity to use this?
No. Room size and dampness alone are enough for a basic capacity + ACH suggestion. Current and target humidity are only used for the rough hours-to-target-RH estimate. If you do not have a hygrometer yet, leave those boxes blank and buy one before relying on any runtime planning.
What if I want to dry more than one room?
Portable dehumidifiers are usually sized for a single room or zone. If you plan to leave doors open and dry several rooms at once, use the calculator with the largest combined floor area and choose a higher dampness level. For whole-home moisture control, a dedicated ducted system or HVAC dehumidifier is often a better match than one portable unit.
How to use this dehumidifier & ACH room sizer
This calculator is meant to turn a few quick tape-measure numbers into a clear starting point: room volume, dehumidifier size in pints/day and litres/day, a simple ACH band, and a rough idea of run time to drop back toward a healthier humidity.
1. Pick units and describe how damp the room feels
Start with the units selector. It loads in feet (ft) for US users, but you can switch to metres if you live elsewhere. Then choose the dampness level that feels closest – slightly damp, damp, very damp or wet. The tool uses this to nudge both capacity and ACH upwards for tougher rooms like basements or laundry areas.
2. Measure length, width and ceiling height
Grab a tape, measure the longest length and width of the area you want to dry, and add the average ceiling height. You do not need whole-house sizes; this is intended for single rooms, half basements, or similar spaces. The calculator turns those numbers into room volume in ft³ and m³ automatically.
3. Optionally add current and target humidity
If you have a hygrometer, add your current RH and a target (many homes aim for about 40–50% RH). The result card will then include a rough hours-to-target-RH window. This is only a planning estimate – always watch the real hygrometer instead of trusting a single number.
4. Read the volume, size, ACH and runtime hints
After you hit Size dehumidifier, you’ll see:
- Room volume in ft³ and m³, plus floor area in ft² and m².
- A suggested dehumidifier size in pints/day and L/day.
- A recommended ACH band and matching airflow in cfm and m³/h.
- If humidity was entered, an approximate runtime range to reach the target RH.
Use this as a comparison tool when you read product boxes, ENERGY STAR charts or retailer sizing tables.
5. Use the copy summary as a simple room note
Tap Copy summary to drop the key numbers into a note or email. Logging a few main rooms (bedroom, living room, basement) makes it easier to pick one or two models that will realistically cover the spaces you care about most.
How the dehumidifier sizing and ACH math works
This tool does not replace detailed manufacturer charts. Instead it uses a small set of transparent rule-of-thumb steps so you can see where the suggested size and ACH band come from and adjust them if you prefer a different safety margin.
1. Turning room size into volume
If you enter dimensions in feet, room volume is:
Volume_ft³ = length_ft × width_ft × height_ft
The calculator converts this to cubic metres using 1 m³ ≈ 35.31 ft³. If you start in metres, the same formulas run in the opposite direction. Floor area in ft² and m² is also calculated so it matches common sizing tables.
2. Rule-of-thumb dehumidifier size from area
Capacity is estimated from floor area and dampness. A simple base rule is:
Base pints/day ≈ floor_area_ft² ÷ 50
That gives about 10 pints/day for 500 ft² and 20 pints/day for 1,000 ft². The calculator multiplies this by a small dampness factor (higher for “very damp” and “wet”), then gently clamps the result into a typical portable range of roughly 10–80 pints/day. Litres/day are shown using 1 US pint ≈ 0.473 L.
3. From volume to an ACH band and airflow
For each dampness level the tool stores a simple ACH range. As an example:
- Slightly damp: about 0.5–0.7 ACH.
- Damp: about 0.7–1.0 ACH.
- Very damp: about 1.0–1.5 ACH.
- Wet: about 1.5–2.0 ACH.
Airflow in cfm (cubic feet per minute) is then:
cfm = Volume_ft³ × ACH ÷ 60
and airflow in m³/h is:
m³/h = Volume_m³ × ACH
The result card shows a low and high airflow that match the chosen ACH band, so you can compare with fan ratings if the manufacturer publishes them.
4. Rough hours-to-target-RH estimate
To keep things simple, the calculator uses a very approximate model for humidity removal. It combines room volume, the suggested L/day capacity and the difference between current RH and target RH into a planning range in hours. Bigger rooms and smaller dehumidifiers give longer times; smaller rooms and larger units give shorter times.
Real drying speed depends on many factors – leaks, cold surfaces, outside weather, hidden damp materials – so this number should always be treated as a rough guide only. Your hygrometer on the wall is the final judge of when you have actually reached a healthier humidity.
Because the maths stays simple and visible, you can repeat the steps by hand, tweak the safety margins, or align the numbers with the sizing charts from your favourite manufacturer or dehumidifier brand.
References and further reading on humidity and dehumidifier sizing
Use this room and ACH sizer alongside trusted humidity and moisture-control resources:
- EPA — A Brief Guide to Mold, Moisture and Your Home — key humidity targets and moisture-control principles for homes.
- EPA Mold Course — Humidity and dampness — explains why keeping indoor RH below about 60% helps limit mold.
- CDC — Mold and dampness in your home — health-focused guidance on humidity, ventilation and cleanup.
- ENERGY STAR — Dehumidifiers — overview of dehumidifier capacity, energy use and buying tips.
- Natural Resources Canada — Operating your house — practical tips on humidity, comfort and energy-efficient operation.
Always combine calculator results with the specific sizing charts and instructions from your dehumidifier’s manufacturer, and consult local building or ventilation professionals for complex or whole-home damp issues.