HVAC sizing — Quick answer
An HVAC calculator estimates the cooling / heating capacity (BTU/h or kW) needed to condition a space, based on floor area, climate, insulation, and internal heat gains.
Rule-of-thumb: BTU/h = floor area × 25 (mild) to 60 (hot/sunny)
Manual J approach: Q = U × A × ΔT + solar + occupant + equipment gains
- floor area — conditioned space (sq ft)
- U × A — envelope conduction loss (W/K)
- ΔT — indoor-outdoor temperature differential (K)
- BTU/h — 1 kW ≈ 3,412 BTU/h; 1 ton refrigeration = 12,000 BTU/h
Worked example: 200 sq ft bedroom, mild climate, average insulation. BTU/h = 200 × 25 = 5,000 BTU/h (0.5 ton, 1.5 kW). Hot sunny upper-floor room: 200 × 50 = 10,000 BTU/h.
HVAC sizing rule of thumb by climate and room type
| Room area | Mild (BTU/h) | Moderate (BTU/h) | Hot/sunny (BTU/h) | Tons |
|---|
| 100 sq ft | 2,500 | 3,500 | 6,000 | 0.25–0.5 |
| 200 sq ft | 5,000 | 7,000 | 12,000 | 0.5–1.0 |
| 400 sq ft | 10,000 | 14,000 | 24,000 | 1.0–2.0 |
| 800 sq ft | 20,000 | 28,000 | 48,000 | 2.0–4.0 |
| 1,500 sq ft | 37,500 | 52,500 | 90,000 | 3.0–7.5 |
| 2,500 sq ft | 62,500 | 87,500 | 150,000 | 5.0–12.0 |
Standard / source: ACCA Manual J (residential load calc); ASHRAE 90.1 (energy efficiency); ISO 13790 (thermal performance).
Used for: AC unit selection, mini-split sizing, ductless heat-pump capacity, commercial RTU sizing, HVAC contractor quotes.
Standards & method
✓ Independently verified 12 July 2026- Governing standard
- ASHRAE Fundamentals β Duct Design
- Clauses applied
- Equal-friction duct sizing Β· ASHRAE recommended friction rate (β0.08β0.10 in.wg per 100 ft) and velocity limits by application Β· circular-equivalent diameter for rectangular ducts
- Core formula
A = Q/v Β· D_eq = 1.30Β·(aΒ·b)^0.625 / (a+b)^0.25- Why this matters
- A rectangular duct of the same cross-sectional AREA as a round one does not have the same capacity β the equivalent-diameter relation accounts for the extra friction, and aspect ratios above ~4:1 waste energy.
- Independently verified
- 12 July 2026 — Re-derived from the governing standard and checked numerically against worked reference cases from the standard itself β not merely tested for βreturns a numberβ.
Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.
HVAC Equation for Duct Sizing
The relationship between Volume of air (CFM), the cross sectional Area of the duct (sq. ft), and the Velocity of the air (FPM) is defined by the basic continuity equation.
Frequently Asked Questions
How do you calculate CFM for a duct?
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CFM is calculated by multiplying the internal Cross Sectional Area of the duct (in square feet) by the Air Velocity (in Feet Per Minute).
What is the formula for HVAC cooling load?
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Total cooling load (kW) = Sensible load + Latent load. Sensible load covers heat from: solar radiation through glass, heat conduction through walls/roofs, people (75W sensible each), lighting (watts Γ 3.41 BTU/hr), equipment, and ventilation air. Latent load covers moisture from occupants (55W/person) and ventilation. ASHRAE Handbook of Fundamentals and Manual J provide detailed calculation methods.
What is the difference between BTU and kW in HVAC?
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BTU (British Thermal Unit) measures heat energy; BTU/hr measures cooling/heating power. Conversion: 1 kW = 3,412 BTU/hr. A 5kW air conditioner equals ~17,060 BTU/hr. In the US, residential HVAC is rated in BTU/hr (range: 5,000β60,000 BTU/hr). Commercial systems are rated in tons of refrigeration: 1 ton = 12,000 BTU/hr = 3.517 kW.
How do I calculate the right air conditioner size?
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A simplified rule of thumb: allow 150β200 BTU/hr (44β59W) per mΒ² of floor area in a typical climate. For a 20mΒ² room: 20 Γ 175 = 3,500W = 3.5 kW. However, accurate sizing requires Manual J or ASHRAE load calculations accounting for climate zone, insulation levels, window area and orientation, occupancy, and local solar intensity. Oversizing causes short-cycling and poor dehumidification.
What is COP and SEER in HVAC equipment?
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COP (Coefficient of Performance) = useful heating or cooling output (kW) / electrical power input (kW). A COP of 3.5 means 3.5 kW of cooling for each 1 kW of electricity. SEER (Seasonal Energy Efficiency Ratio) = total seasonal cooling output (BTU) / total seasonal electrical input (Wh) β it accounts for varying conditions through the season. Minimum SEER ratings in the US are now 14β15 depending on climate region.
What is sensible vs latent heat in air conditioning?
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Sensible heat changes air temperature (measured by thermometer) β caused by solar gain, conduction, equipment, and lighting. Latent heat changes air moisture content (humidity) without changing temperature β caused by people perspiring, cooking, and humid outdoor air. In tropical climates, latent load can be 40β60% of total cooling load. Most packaged air conditioners handle both; in high-humidity climates, dedicated dehumidification or dedicated outdoor air systems (DOAS) may be needed.