Is this a replacement for Manual J?

No. This is a rule-of-thumb estimate. Always verify with a Manual J load calc or licensed HVAC contractor.

Does it handle whole-house loads?

This is for rooms/zones. For whole-home systems, use professional load calculations to account for ducts and infiltration.

What about very sunny glass rooms?

Increase the sun factor or treat the space separately—large glass areas often need higher BTUs than typical rooms.

Cooling only or heating too?

Cooling only. Heating loads depend on climate and equipment; consult an HVAC pro for heating design.

Should I oversize to be safe?

Avoid heavy oversizing; it can cause short cycling and humidity issues. Use this as a starting point, then get a pro sizing.

construction calculator

HVAC BTU Calculator

Estimate cooling BTU and tonnage based on room size, ceiling height, insulation, sun exposure, and occupancy.

Results

Area (sq ft): 300.00
Estimated cooling load: 7200.00 BTU/hr
Recommended tonnage: 0.60 tons

How to use this calculator

Enter room length, width, and ceiling height.
Pick insulation quality and sun exposure to adjust the baseline load.
Add occupants; review total BTU/hr and recommended tonnage.

Inputs explained

Room length/width: Floor dimensions of the cooled space in feet.
Ceiling height: Higher ceilings need more cooling; we scale vs an 8 ft baseline.
Insulation/efficiency: Tighter homes need fewer BTUs; leaky/older homes need more.
Sun exposure: Sunny west/south rooms often need a bump; shaded rooms can drop a bit.
Occupants: Adds ~600 BTU per person beyond two people for body heat and equipment.

How it works

We apply a BTU-per-square-foot rule of thumb adjusted by insulation and ceiling height.

We add ~600 BTU per occupant over two people and adjust for sun exposure (+/- 10%).

Formula

Area = Length × Width
Base BTU = Area × BTU per sq ft × (Ceiling height ÷ 8)
Occupant add = max(Occupants − 2, 0) × 600
Required BTU = (Base BTU + Occupant add) × Sun factor
Tonnage ≈ Required BTU ÷ 12,000

When to use it

Quickly sizing a mini-split or window unit before talking to an HVAC pro.
Checking if an existing AC is undersized for a bonus room or addition.
Comparing loads for sunny vs shaded rooms when planning ductless zones.

Tips & cautions

These are rule-of-thumb estimates. Always confirm final sizing with Manual J or a licensed HVAC contractor.
If you add lots of electronics or cooking heat, increase occupancy/loads accordingly.
For multiple rooms or open layouts, calculate each area and consider combined loads if sharing equipment.
Not a Manual J; does not model infiltration, duct losses, latent loads, or local design temperatures.
Assumes standard residential construction; extreme climates or glass-heavy rooms may need higher factors.
Cooling only—heating loads differ by climate and equipment type.

Worked examples

20×15 room, 8 ft ceiling, average insulation, sunny, 3 occupants

Area = 300 sq ft
Base BTU ≈ 300 × 22 × 1 = 6,600
Occupant add = 600
Sun factor 1.1 → Required BTU ≈ (7,200) × 1.1 ≈ 7,920
Tonnage ≈ 0.66 tons

24×18 bonus room, 9 ft ceiling, poor insulation, shaded, 4 occupants

Area = 432 sq ft
Base BTU ≈ 432 × 28 × 1.125 ≈ 13,608
Occupant add = (4 − 2) × 600 = 1,200
Sun factor 0.9 → Required BTU ≈ (14,808) × 0.9 ≈ 13,327
Tonnage ≈ 1.11 tons

Deep dive

Use this HVAC BTU calculator to estimate cooling load and tonnage from room size, ceiling height, insulation quality, sun exposure, and occupants.

Great for mini-splits or window units as a starting point—then confirm with Manual J or an HVAC pro for final sizing.

FAQs

Is this a replacement for Manual J?: No. This is a rule-of-thumb estimate. Always verify with a Manual J load calc or licensed HVAC contractor.
Does it handle whole-house loads?: This is for rooms/zones. For whole-home systems, use professional load calculations to account for ducts and infiltration.
What about very sunny glass rooms?: Increase the sun factor or treat the space separately—large glass areas often need higher BTUs than typical rooms.
Cooling only or heating too?: Cooling only. Heating loads depend on climate and equipment; consult an HVAC pro for heating design.
Should I oversize to be safe?: Avoid heavy oversizing; it can cause short cycling and humidity issues. Use this as a starting point, then get a pro sizing.

Related calculators

construction

Insulation R-Value Calculator

Estimate insulation thickness needed to hit a target R-value.

finance

AC Cost Calculator

Estimate daily and monthly energy cost for an air conditioner by BTU, SEER, and usage.

construction

Home Generator Size Calculator

Estimate generator size in kW using running load, motor surge, and headroom.

Rule-of-thumb estimate only. Confirm HVAC sizing with a Manual J calculation or licensed HVAC professional before purchasing equipment.