BTU Calculator
The ENERGY STAR room law — 20 BTU per square foot, sun and ceiling adjusted, people and kitchens added, then snapped to a real unit you can actually buy.
BTU Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Room-scale only — whole-house tonnage lives on the HVAC Load page.
- Stock-capacity chart; odd-size units exist but are not charted.
- No heating-mode (heat-pump) sizing.
In short: A 300-ft² sunny kitchen with three regular occupants: 6,000 base × 1.1 sun + 600 third occupant + 4,000 kitchen = 11,200 BTU/h → a 12,000-BTU unit. The quiet 15×12 bedroom: 3,600 → the 5,000 floor unit. Bigger is not better — an oversized AC cools fast, short-cycles and leaves the room clammy.
Formula
base = 20 × ft² × (H ÷ 8) × sun
load = base + 600 × max(0, people − 2) + kitchen
unit = next standard capacity ≥ load
sun: 0.9 shaded · 1 normal · 1.1 sunny
[('20', 'BTU/h per ft², the ENERGY STAR baseline'), ('+4,000', 'the kitchen adjustment'), ('+600', 'per regular occupant past two'), ('snap up', 'to 5/6/8/10/12/14/18/24-k units')]
Worked Example
- Measure the room inside walls; multiply for area.
- Scale for ceiling height and sun exposure.
- Add 600 per person past two; 4,000 for kitchens.
- Snap up to the nearest standard unit.
- Resist the bigger unit — cycles matter.
300 ft² sunny kitchen, 3 people: 6,000 × 1.1 + 600 + 4,000 = 11,200 → 12,000-BTU unit. 15×12 shaded bedroom, 2 people: 3,600 → 5,000 floor unit.
Strengths & Limits Of This Model
Where this engine is strong
- ENERGY STAR adjustments exactly as published
- Snap-up chart — answers in purchasable units
- The oversizing case made on its own card
Where it stops
- No heating mode
- Single-zone scope
Practical Use Cases
Bedroom units
The 5-6k window-box decision.
Home offices
Equipment heat, one occupant.
Sunrooms
The 1.1 sun factor’s home turf.
Kitchen comfort
The +4,000 cooking area.
Mini-split quotes
One-room capacity in hand.
Methodology & Editorial Standards
The ENERGY STAR room law verbatim: base 20 × ft², scaled by H/8 and the sun factor (0.9/1/1.1 chips, 0.7-1.3 band), plus 600 per regular occupant past two and the kitchen add (chip at 4,000, 0-8,000 band). The load snaps UP to the stocked unit chart (5,000-24,000); past the chart the page says 24,000+ / mini-split instead of inventing capacities. Rooms with no area, heights outside 6-14 and kitchen loads past 8,000 refused. The oversizing card is doctrine, not decoration — the snap-up rule stops at the unit, never at the next size.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
BTU Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many BTU do I need per square foot?
About 20 BTU/h per square foot at standard 8-ft ceilings — the ENERGY STAR baseline — then adjust: ±10% for sun, +600 per person past two, +4,000 for a kitchen.
What size AC for a 300 square foot room?
Base 6,000. Very sunny ×1.1, a third regular occupant +600, kitchen +4,000: 11,200 BTU/h → buy the 12,000 unit. Shaded and empty, the same room snaps to 5,000.
Can an air conditioner be too big?
Yes, and it is the common mistake: oversized units short-cycle — cooling the air before removing humidity — leaving rooms cold-clammy and bills 20-30% higher. Slightly small dehumidifies better.
Why add 4,000 BTU for a kitchen?
Cooking appliances dump real heat into the room — the ENERGY STAR adjustment prices it at 4,000 BTU/h. Past 8,000 of kitchen load you are into commercial ventilation territory.
Does ceiling height matter for a room AC?
It scales the air volume: multiply by height ÷ 8. A 10-ft ceiling on the same footprint adds 25% to the base before people and kitchens.
Window unit or mini-split?
Window: $300-1,100 installed, loud, seasonal. Mini-split: $2,000-10,500, SEER 20-35+, modulating — quieter and far more efficient where the room matters.
HVAC Load or BTU page — which one?
One room, one unit, the ENERGY STAR chart: here. The whole house in tons with climate factors and Manual S: the HVAC Load page. Same 20 BTU/ft² grammar, two scales.
Does a big room unit need a special outlet?
Units past about 14,000 BTU typically require 230 V — never on an extension cord. Check the nameplate before you buy, and have the circuit ready before delivery day.