AC and Furnace Sizing Calculator

A rough BTU, tonnage and heat-loss estimate to take to your contractor.

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Cooling load46,200 BTU/h
AC size4 tons
Heat loss39,600 BTU/h
Furnace input41,684 BTU/h
AccuracyA screening estimate. Equipment should be chosen from a Manual J load calculation

Turn these numbers into a build plan.

  • 4.0 tons cooling
  • 39,600 BTU/h heat loss
  • 41,684 BTU/h furnace input

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Project plan · built from your numbers

Plan an HVAC replacement, 1,800 sq ft

A rough estimate of 4.0 tons cooling and 39,600 BTU/h heat loss, to take into a Manual J load calculation and contractor bids.

Materials
$0
Time
2 to 3 weeks of calendar time
Level
Easy
5 steps 3 tools 2 materials
  1. 1

    Gather the house data

    Record floor area, ceiling height, window count, insulation and the current equipment label.

  2. 2

    Ask for a Manual J

    Tell every bidder you want a room-by-room load calculation, not a same-size swap.

  3. 3

    Collect three bids

    Compare them against this estimate of 4.0 tons, and ask about any large gap.

01 · Your numbers
Cooling
4.0 tons
Heat loss
39,600 BTU/h
Furnace input
41,684 BTU/h
02 · The plan

5 steps · 3 tools · 2 materials

2 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

3/5

Plan an HVAC replacement, 1,800 sq ft

Finished in 2 to 3 weeks of calendar time · $0 DIY

Built for your cooling of 4.0 tons. Your first plan saves free. No card needed.

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Borrow these 3 tools from neighbors

How to estimate AC and furnace size

Oversized equipment short-cycles and dehumidifies poorly, so use this to question a quote rather than to buy equipment. Maintenance starts with the filter: see the HVAC filter guide.

  1. Find the area. Use conditioned floor area only.
  2. Estimate cooling. Multiply area by a climate factor, then adjust for ceiling and insulation.
  3. Convert to tons. One ton is 12,000 BTU/h. Round up to the half ton.
  4. Estimate heat loss. Multiply area, design temperature difference and an insulation factor, then divide by AFUE for input.

Frequently asked questions

What size AC do I need for 1,800 sq ft?

In a mixed climate with average insulation this estimate gives about 4 tons. A Manual J calculation often lands lower for a tight house.

Is bigger better for an AC?

No. An oversized unit cycles on and off quickly, leaving the air humid and wearing parts faster.

Can I size a furnace from square feet?

Only roughly. Heat loss depends on climate, insulation and air leakage, so use this as a starting point.

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