TryBuildCalcBasement Dehumidifier Calculator (Below-Grade Sizing)
Size your basement dehumidifier instantly.
🕒 Last updated: September 12, 2026
Inputs
ℹ️Floor area of the space to be conditioned.
ℹ️A taller ceiling means more air volume to condition.
ℹ️How damp the space currently feels/measures — higher levels need more capacity.
Recommended Dehumidifier Size
35 Pints/Day
Next standard retail size at or above the 35 pints/day raw requirement
This result uses the CURRENT (2020+) ENERGY STAR capacity basis. The DOE changed dehumidifier test conditions in 2019 (a cooler 65°F test vs. the old 80°F test), so the same physical unit shows a lower rated capacity under the current standard — per ENERGY STAR's own published conversion, a legacy-rated 70 pint unit is roughly equivalent to a 40-45-pint unit under the current standard. If a product you're considering was rated before 2020 (or its listing doesn't say), its number will look higher than this result for the same real performance — confirm which standard it uses before comparing.
Space
Area: 800 sq ft
Ceiling height: 8 ft
Dampness level: Very Damp (75-90% RH)
Conditions
Below grade/basement
Sizing Breakdown
Base (area & dampness): 30 pints/day
Height multiplier: x1
Condition add-ons: +5 pints/day
Assumptions Used
Base pints/day follows ENERGY STAR's own current dehumidifier buying guide directly (energystar.gov/products/dehumidifiers), which publishes 3 dampness bands (Slightly-to-Moderately Damp, Very Damp, Wet) with a published capacity range for spaces under and over 2,000 sq ft. This calculator turns that into a continuous formula — anchored at 300 sq ft (the range's low figure) and 2,000 sq ft (its high figure) — rather than treating any area beyond a fixed cutoff as an unchanged, silently-reused number, and floors the result at the officially-stated low figure for very small spaces. That base is scaled by a ceiling-height multiplier (1 + 0.125 x feet above 8 ft — an 8 ft room uses x1.0, a 12 ft room x1.5) and increased by flat add-ons commonly cited alongside AHAM-style sizing guidance (a separate convention from ENERGY STAR's own table): below-grade/basement space (+5), no vapor barrier on the floor (+5), regular occupancy by 3+ people (+5), a nearby laundry/dryer (+5), and a nearby indoor pool/hot tub (+15, the midpoint of that source's cited 10-20 range). The combined raw figure is rounded UP to a standard retail tier (22, 30, 35, 50, or 70 pints/day) — never down, and the SAME rounded-up number is used both for display and for choosing the tier, so the two can never look inconsistent with each other. A space too large or damp for any single portable tier is flagged directly rather than guessed at. This is a planning estimate — always verify your chosen product's actual rated capacity AND which DOE test-condition standard (pre- or post-2019) that rating uses before purchasing.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Humidifier / Dehumidifier Size Calculator.
Basement dehumidifier sizing
Basements are the single most common dehumidifier use case — below-grade contact with cooler surrounding soil raises baseline humidity, which the Below Grade toggle adds 5 pints/day to account for.
This page defaults to an 800 sq ft, very damp basement with the Below Grade toggle on — edit the inputs above to match your actual basement.
Humidifier / Dehumidifier Sizing Formula: How Is It Determined?
Humidifier mode uses a volume-based load formula from an ARI/HARDI industry standard. Dehumidifier mode follows ENERGY STAR's own current dehumidifier buying guide, turned into a continuous area/dampness formula with adjustment factors.
Humidifier Mode
Volume (cu ft) = Area (sq ft) x Ceiling Height (ft)
Raw GPD = (Volume x K) / 33,000, minus 2 if occupant credit applied (floored at 0)
K = 17.5 (tight) / 27 (average) / 38 (loose)
Per HARDI Tech Tip #6 (derived from ARI Standard 630-74), K is a moisture-infiltration factor tied to how many air changes per hour a home's construction lets in. The raw requirement is rounded UP to a standard tier for your selected Humidifier Type — Evaporative: 12, 17, 18, or 21 gal/day; Steam: up to 34.6 gal/day (Aprilaire's Model 800, the common whole-house steam tier) — since evaporative and steam units are different technologies with very different real capacity ranges.
Dehumidifier Mode
Base Pints = ENERGY STAR anchor points for the dampness level, interpolated/extended by area
Height Multiplier = 1 + 0.125 x max(0, Ceiling Height - 8)
Raw Pints = (Base Pints x Height Multiplier) + Condition Add-ons
ENERGY STAR's own current buying guide gives a low/high pints-per-day figure for each dampness level at 300 sq ft and 2,000 sq ft (its own bracket boundary) — this calculator treats those as two points on a line and extends the same rate beyond 2,000 sq ft, rather than treating any area past a fixed cutoff as an unchanged, stale number. A taller ceiling scales the load up proportionally; below-grade space, missing vapor barriers, higher occupancy, nearby laundry, and a nearby pool/hot tub each add a flat amount (a separate, commonly-cited convention, not from ENERGY STAR's own table). The combined figure is rounded UP to a standard retail tier (22, 30, 35, 50, or 70 pints/day).
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Area / ceiling height | 800 sq ft / 8 ft |
| Dampness level | Very Damp (75-90% RH) |
Sizing Steps
| Calculation | Result |
|---|---|
| Base pints (ENERGY STAR area/dampness formula) | 30 pints/day |
| Height multiplier | x1 |
| Condition add-ons | +5 pints/day |
| Raw requirement | 35 pints/day |
| Nominal size, rounded up | 35 pints/day |
Therefore, this space needs a dehumidifier rated at approximately 35 pints/day. This result uses the current (2020+) ENERGY STAR capacity basis — confirm whether your chosen product's rating uses the same basis before comparing.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.