HVAC Calculators
Size a system, price what it costs to run, and check what a change would actually save. Five tools that compute as you type, plus every calculator on the site in one place.
Estimates, not engineering. Every tool shows the formula it used and where the numbers come from. For anything you are going to spend money on, get a contractor to run a Manual J.
Every tool on the site
Grouped by the question you are actually trying to answer.
Sizing and load
Getting the size right is the single highest-leverage HVAC decision. Oversized equipment short-cycles, never removes humidity, and wears out early.
Cost and savings
What your system costs to run today, and what changing something would actually save.
Refrigerant
For the situation where a technician has told you the system is low on refrigerant and you have no way to judge what you just heard.
Filters and airflow
The cheapest part of the system, and the one most often behind symptoms that get misdiagnosed as something expensive.
How these calculators work
Each tool states its formula in the result, because a number without a method behind it is not worth much. The three that do the heavy lifting:
- Cooling load starts from a 20 BTU per square foot baseline and applies multipliers for ceiling height, climate, sun exposure, and insulation, plus 600 BTU for each occupant beyond two. That is a screening estimate. A Manual J calculation is the real answer and is what a contractor should be sizing from.
- Running cost uses kW = BTU ÷ SEER ÷ 1000, then multiplies by run hours and your electricity rate. The rate defaults to 18.34 cents per kWh, the US residential average for June 2026 published by the EIA, but your own rate from a recent bill will always be more accurate.
- Thermostat setback scales the Department of Energy guidance that a 7 to 10°F setback for 8 hours a day saves up to 10 percent a year. It caps at 15 percent, because the relationship stops being linear at large setbacks and pretending otherwise would overstate the case.
The filter rating converter uses 3M’s own published MERV-to-MPR chart, which is why MPR values are marked verified while Home Depot’s FPR values are marked approximate — Home Depot publishes only a coarse four-level scale, not a numeric conversion table.
Where these tools stop
These estimate and compare. They do not tell you how to work on a sealed refrigerant circuit, wire line voltage, or touch a gas connection. Refrigerant work in particular requires EPA Section 608 certification under federal law, with no homeowner exemption.
Frequently asked questions
Are these HVAC calculators free?
Yes. Every tool on this page is free, needs no account, and runs entirely in your browser. Nothing you type is transmitted or stored.
How accurate is an online AC sizing calculator?
Good enough to sanity-check a contractor quote, not good enough to size equipment from. A proper Manual J load calculation accounts for room-by-room orientation, window specification, duct losses, and infiltration. Treat any square-footage rule of thumb, including ours, as a starting point.
What electricity rate should I use?
Your own, from a recent bill — divide the total by the kWh used and you get your true blended rate including delivery charges. The default here is 18.34 cents, the US residential average for June 2026 from the EIA.
Why does an oversized air conditioner cool worse?
It cools the air fast, hits the setpoint, and shuts off before it has run long enough to pull moisture out. You get a cold clammy house, short-cycling, and a compressor that wears out early. Correct sizing matters more than extra capacity.
Do these calculators work on a phone?
Yes. The tools recalculate as you type and are built to be usable one-handed during a service visit.
Sources & verification
✓ Verified against a primary source
- Default electricity rate of 18.34 cents per kWh is the US residential average for June 2026 — EIA, Electric Power Monthly, Table 5.6.A, eia.gov
- Thermostat setback savings scale the DOE guidance of up to 10% a year from a 7–10°F setback for 8 hours daily — US Department of Energy
- MERV to Filtrete MPR conversions come from 3M’s own published comparison chart — 3M Filtrete
⚠ Provisional — approximate, not a single authoritative table
- Home Depot FPR values are consensus ranges. Home Depot publishes a coarse four-level scale rather than a numeric FPR-to-MERV table.
- The 20 BTU per square foot sizing baseline is an industry screening heuristic, not a manufacturer specification. Manual J is the authoritative method.
Last reviewed 2026-09-01. How we verify what we publish →