Humidity Control Calculator — Free Online Calculator
Whether you are pulling moisture out of a damp basement or adding humidity to dry winter air, you need the latent load in BTU/h and pints or gallons per day — not a guess from square footage alone. Enter floor area, ceiling height, air changes, dry-bulb temperature, and current vs target RH; this tool converts RH to grains, calculates CFM and latent load, and sizes dehumidifier or humidifier capacity instantly.
Enter space and humidity targets
Dehumidify when current RH is above target; humidify when it is below.
Conditioned floor area or zone being sized.
Average ceiling height for volume calculation.
How often the air volume turns over per hour.
Space temperature for psychrometric conversion.
Measured or estimated RH in the space today.
Desired RH — typically 45–50% in summer, 35–40% in winter.
Moisture removal capacity
Latent load
17,916 BTU/h
Airflow (CFM)
1,333 CFM
Moisture difference (Δgrains)
20 gr/lb
What this means
Whole-home range — Typical whole-home or large basement load — look at 50–70 pint/day rated units or ducted dehumidification.
See the breakdown
Sizing shortcut from psychrometric targets — not a substitute for Manual J or manufacturer test conditions. Verify rated capacity against AHRI dehumidifier ratings and actual field RH readings.
Dehumidifier sizing at common RH targets
2,000 sq ft × 8 ft ceiling · 5 ACH · 75°F dry-bulb. Same space — only the RH change varies.
| Current → target RH | Δgrains | Latent load | Capacity needed |
|---|---|---|---|
| 65% → 50% | 20 gr/lb | 17,916 BTU/h | 36 pints/day |
| 70% → 50% | 26 gr/lb | 23,923 BTU/h | 48 pints/day |
| 60% → 45% | 20 gr/lb | 17,862 BTU/h | 36 pints/day |
| 55% → 45% | 13 gr/lb | 11,891 BTU/h | 24 pints/day |
Sources & standards: ASHRAE Handbook — Fundamentals (psychrometrics and latent load) · AHRI Standard 920 (dehumidifier performance ratings) · ACCA Manual J (whole-building load methodology).
The formula, explained in plain English
Humidifier and dehumidifier sizing starts with how much moisture you need to add or remove per hour. Relative humidity tells you the starting and ending moisture state; room volume and air changes set how much air carries that load.
CFM from ACH
Air changes per hour express how fast the space air turns over. Multiply room volume by ACH and divide by 60 to get CFM — the airflow basis for the latent-load equation.
Grains from RH
Current and target RH convert to grains per pound of dry air at the same dry-bulb temperature using the Magnus approximation. The difference (Δgrains) is the moisture change the equipment must handle.
0.68 × CFM × Δgrains
The 0.68 factor is the standard latent-load constant for grains and CFM at typical indoor conditions. Result is Ql in BTU/h — the same latent quantity used in coil and ventilation load calculations.
Pints vs gallons
Dehumidifiers are rated in pints/day (AHRI 920); humidifiers in gallons/day. Divide BTU/h latent load by 500 (12,000 ÷ 24) to get pints/day — then divide by 8 for gallons when humidifying.
Worked examples
Three humidity-control scenarios — a whole-home dehumidify, a basement dry-out, and a winter humidify job.
Whole-home dehumidify — 2,000 sq ft, 65→50% RH
CFM = 16,000 × 5 ÷ 60 = 1,333 CFM
Δgrains (75°F, 65→50% RH) = 20 gr/lb
Ql = 0.68 × 1,333 × 20 = 17,916 BTU/h
pints/day = 17,916 × 24 ÷ 12,000 = 36 pints/day
Result: typical clammy summer home — a 50–70 pint/day whole-home or ducted dehumidifier class. Pair with the cooling coil if it already handles part of the latent load.
Basement dry-out — 1,200 sq ft, 70→50% RH, 3 ACH
CFM = 9,600 × 3 ÷ 60 = 480 CFM
Δgrains (70°F, 70→50% RH) = 22 gr/lb
Ql = 0.68 × 480 × 22 = 7,234 BTU/h
pints/day = 7,234 × 24 ÷ 12,000 = 14 pints/day
Result: portable range — a 20–30 pint/day class unit may cover this zone. Lower ACH reflects a partially sealed basement; increase ACH if the HVAC blower circulates the whole volume.
Winter humidify — 2,000 sq ft, 25→40% RH, 2 ACH
CFM = 16,000 × 2 ÷ 60 = 533 CFM
Δgrains (68°F, 25→40% RH) = 15 gr/lb
Ql = 0.68 × 533 × 15 = 5,546 BTU/h
gal/day = (5,546 × 24 ÷ 12,000) ÷ 8 = 1.4 gal/day
Result: low winter moisture addition — a bypass or small steam humidifier may suffice. Verify furnace CFM can carry the vapor without condensing in ducts; check the Dew Point Calculator on supply-air conditions.
Frequently asked questions
Common questions about dehumidifier sizing, humidifier capacity, latent load, and indoor RH targets.
What size dehumidifier do I need for a 2,000 sq ft home?
Start with the moisture you need to remove, not just square footage. At 2,000 sq ft × 8 ft ceiling, 5 ACH, 75°F, and 65→50% RH, this calculator shows about 36 pints/day latent load — a whole-home or large basement unit in the 50–70 pint/day class. Tighter homes with lower ACH need less; damp basements with higher RH need more.
How is dehumidifier capacity calculated?
This tool uses the standard latent-load shortcut: Ql = 0.68 × CFM × Δgrains, where CFM comes from room volume and air changes per hour, and Δgrains is the moisture difference between current and target RH at the same dry-bulb temperature. Pints per day = Ql × 24 ÷ 12,000. AHRI-rated pint/day labels on retail units are tested at specific conditions — use this as a sizing starting point, then verify with manufacturer data.
What indoor humidity should I target?
ASHRAE comfort guidance recommends 30–50% RH in occupied spaces. Below 30% feels dry and can crack wood; above 60% feels clammy and supports mold growth. In winter, 35–45% is common with humidification; in summer, 45–55% is often acceptable if the cooling coil handles latent load.
When do I humidify vs dehumidify?
Select Dehumidify when current RH is above your target — summer basements, post-flood drying, or oversized cooling that leaves the space clammy. Select Humidify when current RH is below target — dry winter air in cold climates. The same latent-load formula applies; results show pints/day for removal and gallons/day for addition.
How does ACH affect dehumidifier sizing?
Air changes per hour set how much air passes through the space being conditioned. Higher ACH means more CFM and higher latent load for the same RH change — common for basements (3–5 ACH) or when the HVAC blower circulates whole-home air (5+ ACH). A sealed room with minimal airflow needs a lower ACH input.
Does this replace a full Manual J load calculation?
No — this is a field sizing shortcut for humidifier or dehumidifier capacity from psychrometric targets. It does not account for infiltration, ventilation makeup air, occupant moisture generation, or coil-only latent removal. Use it to ballpark equipment class and quote scope; confirm with a full load analysis or manufacturer sizing tools on larger jobs.
How do grains relate to relative humidity?
Grains of moisture per pound of dry air (gr/lb) express absolute humidity — the actual water mass in the air. RH is relative to saturation at that temperature. Two air samples at the same RH but different dry-bulb temperatures carry different grain counts. This calculator converts RH to grains using the Magnus approximation — same method as the Dew Point Calculator.
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