To size a dehumidifier correctly, you need two numbers: your room's square footage and its moisture severity level. A moderately damp 1,000 sq ft basement needs a 25-pint unit; the same room with visible water seepage needs 45 pints. Use the AHAM sizing chart below or our calculator to get your exact recommendation.
Undersizing is the most common dehumidifier mistake homeowners make — and the most expensive. A unit that's too small runs 24/7, never reaches your target humidity, burns out in 2–3 years instead of 7–10, and costs more in electricity than a properly sized unit.
Dehumidifier Sizing Calculator
The Official AHAM Sizing Chart
AHAM (Association of Home Appliance Manufacturers) publishes the industry-standard sizing guidelines. Their chart accounts for room size and four moisture condition levels:
These ratings are measured at AHAM standard conditions: 65°F and 60% RH. In real-world conditions — especially in warm, very humid environments — actual moisture removal may be 20–40% higher than the rated capacity.
Step 1: Determine Your Moisture Condition Level
The moisture condition level is the most important variable. Here's how to assess yours:
Moderately Damp (50–60% RH)
- Musty smell only during humid weather
- No visible moisture on walls or floors
- Space feels slightly stuffy in summer
- Hygrometer reads 50–60% on humid days
Very Damp (60–70% RH)
- Musty smell present most of the time
- Damp spots occasionally appear on walls or floors
- Condensation on cold-water pipes
- Hygrometer consistently reads 60–70%
Wet (70–80% RH)
- Walls or floors feel damp to the touch
- Visible sweating on walls and windows
- Small puddles appear after rain
- Mold spots visible on surfaces
- Hygrometer reads 70–80%
Extremely Wet (80%+ RH)
- Standing water or consistent flooding
- Active water seepage through walls or floor
- Extensive mold growth on multiple surfaces
- Laundry hung in the space doesn't dry
- Hygrometer reads 80%+ regularly
If your space is "extremely wet," fix the water source before buying a dehumidifier. No portable dehumidifier can overcome active flooding or continuous water intrusion. Address foundation cracks, improve exterior grading, repair leaking pipes, and install a sump pump first. Then size your dehumidifier for the residual moisture.
Step 2: Measure Your Room
Calculate the total square footage of the contiguous space you want to dehumidify. For basements, use the entire footprint even if parts are unfinished.
Standard rooms: Length × Width = Square footage L-shaped rooms: Break into two rectangles, calculate each, and add them together Open floor plans: Include all connected areas without doors
Ceiling Height Adjustment
The AHAM chart assumes standard 8-foot ceilings. If your ceilings are taller, you have more air volume to dehumidify. Adjust like this:
Step 3: Apply Adjustment Factors
Beyond ceiling height, several conditions increase your dehumidification needs:
The Complete Sizing Formula
Required capacity = AHAM chart value × ceiling height factor + adjustment factors
Example calculation:
- Room: 1,500 sq ft basement, 9-ft ceilings
- Condition: Very damp (60–70% RH)
- AHAM chart: 35 pints
- Ceiling adjustment: 35 × 1.13 = 39.6 pints
- Below grade: +5 pints
- No vapor barrier: +5 pints
- Total needed: ≈50 pints
Sizing Recommendations by Space
Real-World Sizing Examples
Example 1: New Townhome Basement in Raleigh, NC (900 sq ft)
Conditions: 8-ft ceilings, moderately damp (55% RH in summer), concrete slab with no vapor barrier, below grade. AHAM chart: 900 sq ft, moderately damp → 20 pints Adjustments: Below grade (+5), no vapor barrier (+5) = +10 pints Total needed: 30 pints Recommendation: A 35-pint portable gives comfortable headroom. A 50-pint unit would work even better and cost roughly the same to operate due to shorter runtimes.
Example 2: Finished Basement in Chicago, IL (1,800 sq ft)
Conditions: 9-ft ceilings, very damp (65% RH, musty smell), below grade, used as a family room with 4 people regularly. AHAM chart: 1,800 sq ft, very damp → 40 pints Adjustments: Ceiling (×1.13 = 45), below grade (+5), 3+ people (+5) = 55 pints Total needed: 55 pints Recommendation: A 50-pint unit is the minimum; a 70-pint portable is more appropriate. For a finished living space of this size, a whole-house dehumidifier ducted into the HVAC system is the best long-term solution.
Example 3: Crawl Space in Savannah, GA (1,200 sq ft)
Conditions: 4-ft ceiling (crawl space), extremely wet (85%+ RH), dirt floor partially covered with vapor barrier, hot and humid climate. AHAM chart: 1,200 sq ft, extremely wet → 45 pints Adjustments: Ceiling height is half standard (but crawl space moisture load is extreme): +10 for exposed dirt, +5 for below grade = 60 pints minimum Total needed: 60+ pints with low-temperature capability Recommendation: Santa Fe Compact70 or similar crawl-space-rated unit. Must be rated to operate at temperatures as low as 49°F. Encapsulate the crawl space with a full vapor barrier for maximum effectiveness.
Example 4: Open-Plan Apartment in Miami, FL (1,100 sq ft)
Conditions: 10-ft ceilings, very damp (68% RH despite running AC), kitchen and bathroom contribute significant moisture, multiple exterior doors. AHAM chart: 1,100 sq ft, very damp → 30 pints Adjustments: Ceiling (×1.25 = 38), frequent door opening (+5), kitchen moisture (+5) = 48 pints Total needed: 48 pints Recommendation: A 50-pint ENERGY STAR portable is ideal. Set it to 48% and let it cycle automatically. Place near the kitchen/bathroom end of the apartment where most moisture originates.
Common Sizing Mistakes
Mistake 1: Trusting the "Room Size" on the Box
Manufacturers often list "up to 4,500 sq ft" on a 50-pint dehumidifier box. That's the maximum area for moderately damp conditions with 8-ft ceilings. For a wet basement with tall ceilings, that same unit might only adequately cover 1,500 sq ft. Always use the AHAM chart with your actual moisture conditions.
Mistake 2: Sizing Based on Old (Pre-2020) Standards
In 2020, the DOE changed how dehumidifier capacity is tested. The new standard tests at 65°F/60% RH instead of the old 80°F/60% RH. A unit rated "70 pints" under the old standard is rated approximately "50 pints" under the new standard. If you're comparing old recommendations to new products, you'll undersize by about 30%.
Mistake 3: Ignoring Air Leakage
A perfectly sized dehumidifier will struggle if the space has massive air infiltration. Every cubic foot of humid outdoor air that leaks in adds to the moisture load. Seal air leaks — especially in basements (rim joists, window frames, duct penetrations) — before finalizing your sizing.
Mistake 4: Using Multiple Small Units Instead of One Right-Sized Unit
Two 25-pint dehumidifiers don't equal one 50-pint in practice. The 50-pint unit has a larger fan, better air circulation, and higher efficiency (IEF). Two small units use more combined electricity, create more noise, and require two buckets to empty. One properly sized unit is always better.
Mistake 5: Sizing for Current Conditions, Not Worst Case
Your basement might read 58% RH today, but what about after three days of rain in August? Size for the worst conditions you expect, not the average. A unit that handles the worst days will easily manage normal days with shorter runtimes.
- Use the AHAM sizing chart as your starting point: room size × moisture severity = base pint rating
- Adjust upward for tall ceilings (multiply by ceiling height factor), below-grade spaces (+5 pints), missing vapor barriers (+5–10 pints), and high-occupancy rooms (+5 pints)
- Always size up, not down — a 50-pint unit in a 35-pint space runs shorter cycles and costs less to operate
- Fix water intrusion and seal air leaks before relying on a dehumidifier
- Post-2020 capacity ratings are ~30% lower than pre-2020 for the same performance — don't confuse old and new standards
Frequently Asked Questions
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