These tools give estimates from common sizing rules. Always confirm big purchases and anything involving ducts, gas, refrigerant or wiring with a pro.
Free calculators for air, HVAC and home comfort. Pick one below, enter your numbers and get a size to shop for.
Table of Contents
Free indoor air, HVAC and home comfort calculators. Real numbers instead of guesswork.
Pick a tool. Plug in your room details. Get a straight answer in seconds, plus real products that meet the spec.
🌬️ Air Purifiers: Size a purifier by CADR. See what filters really cost each year.
❄️ HVAC: Find the right BTU, tonnage and SEER2 savings for homes and businesses.
🔥 Heating: Match the wattage of a space heater to your room.
💧 Humidity and Water: Size a dehumidifier or humidifier. Pick a water softener. Check water hardness by state.
Every calculator is free and runs right in your browser. Test a room, a home or a budget before you spend a dollar.
👇 Choose a group below and start calculating: Air Purifier, HVAC, Heating, Humidity Control, or Water Filtration.
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Which Calculator Should You Use?
- Air purifier size: the CADR calculator. Pair it with how to choose the right size air purifier.
- One room AC: the BTU calculator. Whole house: the AC tonnage calculator. Pair them with how to choose the right size air conditioner.
- Space heater: the wattage calculator, with how to choose the right size space heater.
- Humidifier: the GPD calculator, with how to choose the right size humidifier.
- Dehumidifier: the pint calculator, with how to choose the right size dehumidifier.
- Water softener: the grain calculator. The water hardness by state page helps you guess your hardness before you test.
- Energy savings or commercial buildings: the SEER2 savings and commercial tonnage calculators.
Here is where home sizing tools are heading: tools that pull in your local climate and electric rate, and quotes that start from a load calculation instead of a rule of thumb. More makers now publish test data, so calculators can lean on real ratings.
Get Better Answers From Any Calculator
- Measure the room instead of guessing. Length times width gives floor area, and ceiling height changes the volume.
- Use your real climate, insulation and electric rate, not a best-case number.
- If you are between two sizes, check the recommendation for the room as it is used: sunny, crowded, a kitchen or a bathroom.
- Do not jump more than one size up. Too big is as costly as too small.
How the Math Works
Every calculator here uses a simple, public rule. Purifiers use room volume times air changes per hour, divided by 60. Air conditioners use a floor-area chart with adjustments for sun, people and ceiling height. Heaters use watts per square foot. Softeners use people times daily water use times hardness. You can do the math by hand, and each calculator page shows the formula and worked examples.
Where Calculators Stop
A calculator cannot replace a load calculation, a water test or an electrician. Treat the output as a size to ask about. A contractor should confirm anything that touches ducts, gas, refrigerant, wiring or plumbing.
The downside is false precision. A calculator gives a clean number, but it cannot see leaks, sun, ducts or how you live, and a number to the last digit can feel more certain than it is. Rules of thumb also differ between makers, so two tools can disagree. Expect more people to buy a size straight from a result and skip the checks. Use each answer as a starting point, measure your room twice and ask a pro to confirm anything that involves ducts, gas, refrigerant or wiring.
More Guides on the Site
Looking for the cost of upkeep? See air purifier filter replacement cost. Want to compare picks after you know your size? The category hubs cover air purifiers, air conditioners, humidifiers and dehumidifiers.
FAQ
Are these calculators free?
Yes. Enter your numbers and the results appear on the page.
How accurate are they?
They use common sizing rules, so they are good for a starting size. A pro load calculation is more exact.
Which one should you start with?
Pick the one for the product you plan to buy, then read the sizing guide linked on the same page.
Do the calculators show products?
Many list real models that clear your result. Prices and ratings change, so check each listing.
Final Word
Choose the calculator that fits the project, measure carefully and treat the answer as the start of your shopping, not the end.
What comes next: more regional data, better ratings and tools that link straight to current models. Start with the calculator that matches your project, then read its guide.
Related Guides
Keep going with how to choose the right size air purifier, how to choose the right size air conditioner, how to choose the right size space heater, how to choose the right size humidifier and how to choose the right size dehumidifier.
