About SizeSums
SizeSums answers one question before you buy home equipment: what size do I need where I live?
Most sizing charts use square feet alone. But a house in Houston and the same house in Seattle need very different air conditioners, and a tankless water heater in Minnesota has to heat much colder water than one in Florida. Enter your ZIP code and every SizeSums calculator uses your county's climate zone and design temperatures, then gives a size range and the nearest sizes you can actually buy.
What makes it different
- Your climate, not a national rule of thumb. Every result starts from your county's IECC climate zone (U.S. Department of Energy) and design temperatures computed from NOAA weather station records, shown under the result.
- Your numbers, if you have them. Know your design temperatures from a contractor's report? Type them in and every result updates.
- The math, shown. Every calculator explains its formula, its assumptions and a worked example, so you can check the answer by hand or with your contractor.
The calculators
- AC size calculator: tons and btu for central air in your climate
- Furnace size calculator: input and output btu at 80% and 95% afue
- Boiler size calculator: hot water boiler btu by zip, standard or condensing
- Heat pump size calculator: heating and cooling loads, balance point, backup heat
- Mini split size calculator: btu per room and multi-zone total
- Wood stove size calculator: btu for the space it heats, by zip
- Garage heater size calculator: watts or btu for a garage, by insulation and zip
- Room BTU calculator: window ac, portable ac or space heater for one room
- Generator size calculator: running and starting watts, portable or whole-house
- Power station size calculator: battery wh and inverter watts for an outage
- Wire gauge & breaker calculator: wire gauge, breaker and voltage drop by run length
- EV charger circuit calculator: breaker, wire and run length for a level 2 charger
- Propane tank size calculator: tank gallons for heating and hot water, by zip
- Insulation R-value calculator: attic, wall and floor r-value by zip
- Water heater size calculator: tank gallons, first-hour rating or tankless gpm
- Solar panel size calculator: system kw and panels for your use and sun
- Pool heater size calculator: gas or heat pump btu and heat-up time
- Air purifier size calculator: smoke cadr for your room, everyday or wildfire
- Dehumidifier size calculator: pints per day by room size and dampness
- Humidifier size calculator: gallons a day for the whole house, by zip
- Water softener size calculator: grains by hardness, iron and household
- Well pressure tank size calculator: gallons and drawdown for your pump
- Attic ventilation calculator: ridge and soffit vent by code, by zip
- Gas pipe size calculator: natural gas or propane line by btu and length
- Electrical service size calculator: 100, 150 or 200 amps by nec load calc
- Expansion tank size calculator: thermal expansion tank for a water heater
- Pool pump size calculator: flow (gpm) and pipe size from turnover
- Box fill calculator: electrical box size by nec 314.16
- Whole-house ventilation calculator: fresh-air cfm for an hrv or erv
- Duct size calculator: round and rectangular duct by cfm
- Bath & range hood fan calculator: cfm for a bathroom fan or kitchen range hood
- Ceiling fan size calculator: blade span and downrod for any room
See where our numbers come from for the data sources and formulas behind every result.
Estimates, not a load calculation
Results are estimates for planning and for talking to installers. For central air conditioning, furnaces and heat pumps, a contractor's Manual J load calculation of your actual house is the final word. Electrical work must follow your local code and be inspected where required.
Sister sites
Once you know the size, WattSums shows what the equipment costs to run at your state's electricity rate. For materials around the house and yard, YardSums works out how much concrete, insulation, gravel or paint you need.
Who runs SizeSums
SizeSums is published by Data Dingo, an independent publisher that also runs YardSums and WattSums. We aren't a store, a manufacturer or a contractor: no company pays to be mentioned, and we don't earn commissions on anything you buy.
How we build and check every calculator
- Every number has a source. The formulas and figures come from published references: ACCA's Manual J and Manual D methods, the International Residential and Fuel Gas Codes, the National Electrical Code, ENERGY STAR and DOE rules, NOAA climate data and manufacturers' published sizing charts. Each page names its sources, and our How we calculate page links them.
- Checked against the source. Where a source gives a worked example or a table, we check that the calculator reproduces it, and the page tells you where our method simplifies or differs.
- Tested on every change. Automated tests recheck each calculator's results, and every page is checked on a phone-sized screen, whenever we change a calculator.
- Reviewed and dated. Each page shows the month it was last reviewed.
- Corrected when wrong. If you find a mistake, tell us: we check it against the source and fix it.
Get in touch
Spotted a mistake, or want a calculator we don't have yet? Email hello@sizesums.com or use the contact page.