Insulation & HVAC Calculators

Insulation, BTU sizing and ventilation.

Heating, cooling and insulation are sized from area, climate and how much heat leaks through the envelope. The calculators here cover the two sides of that problem: how much insulation to buy for a given R-value, and how many BTUs of heating or cooling a space needs once it is insulated.

The sizing tools use the rules of thumb contractors quote (20 to 35 BTU per square foot for cooling, 30 to 60 for heating by climate zone) plus a simple heat-loss calculation for a room. They are for planning and comparing quotes; a Manual J load calculation by an HVAC contractor is how equipment is actually sized.

How much material do I need

Quantities to order, in the units a supplier sells them in. 10 calculators.

Insulation CalculatorCalculate how many packages and batts of fiberglass insulation you need for a wall, floor or attic from the area, framing spacing, R-value and coverage.Spray Foam CalculatorCalculate how many board feet of spray foam you need for a wall, ceiling or crawlspace, the R-value at that thickness, and how many 600 or 200 board foot kits to buy.Blown-In Insulation CalculatorCalculate how many bags of blown-in cellulose or fiberglass you need to bring an attic to R-38, R-49 or R-60, plus the depth to blow and the coverage per bag.Mini Split CalculatorCalculate what size ductless mini split head a room needs in BTU from its area, climate, sun, insulation and occupants, with sizing for winter heating and multi-zone use.Heat Loss CalculatorCalculate a room's heat loss in BTU per hour through walls, ceiling, floor, windows and air leakage from R-values, U-factor and design temperatures to size a heater.Attic Ventilation CalculatorCalculate the attic ventilation you need under the IRC 1/300 or 1/150 rule in square inches of net free area, split into soffit intake and ridge exhaust vents.Firewood CalculatorMeasure a delivered stack in cords, check what a face cord really is, and see how many cords a winter needs: a firewood calculator for buyers and burners.Ductwork CalculatorSize supply trunks, branch runs and the return with this ductwork calculator: enter tons, friction rate and duct material to get diameters, velocity and grille area.Propane Usage CalculatorSee how many gallons of propane an appliance burns a day, how long a tank lasts and what a month costs. A propane usage calculator for 20 lb to 1,000 gal tanks.Radiant Floor Heating CalculatorPlan a heated floor with this radiant floor heating calculator: PEX length, loop count, manifold ports, electrical load and the BTU per hour the floor can deliver.

What size do I need

Sizing and specification, against the code or standard that governs it. 11 calculators.

BTU CalculatorCalculate how many BTUs of air conditioning a room needs from its size, climate, sun exposure and occupants to size a window unit or mini-split.Furnace Size CalculatorCalculate what size furnace a house needs in BTU per hour from its square footage, climate zone, insulation and ceiling height, matched to standard 80% or 95% AFUE sizes.Dehumidifier Size CalculatorFind what size dehumidifier you need in pints per day from the room size and how damp it is, using the Energy Star sizing guidance and the 2019 DOE test ratings.Water Heater Size CalculatorSize a water heater by first hour rating from the showers, baths, dishwasher and laundry loads in your busiest hour, with gas, electric and tankless recommendations.R-Value CalculatorBuild a wall, ceiling or floor from layers and this R value calculator returns the nominal R, the whole-assembly R after framing, and the IECC minimum for your zone.Range Hood CFM CalculatorSize kitchen exhaust with this range hood CFM calculator: cooktop width and burner BTU set the airflow, then the duct run and the 400 CFM makeup air rule set the rest.Bathroom Fan CFM CalculatorSize a bath exhaust fan with this bathroom fan CFM calculator: the area rule, the HVI fixture rule, and the duct loss that makes delivered CFM lower than the label.AC Size CalculatorUse this AC size calculator to size whole-house central air in tons from conditioned square footage, IECC climate zone, ceiling height, insulation and sun exposure.Heat Pump Size CalculatorThis heat pump size calculator sizes a whole-house system in tons, then checks heating capacity at 47 F and 17 F, the balance point and the backup heat needed.Boiler Size CalculatorThis boiler size calculator finds a home's hydronic heat loss in BTU per hour, matches it to a standard boiler input and output, and sizes the baseboard or radiators.Ceiling Fan Size CalculatorFind the blade span, downrod length and mounting a room needs. This ceiling fan size calculator works from floor area, ceiling height and where the fan hangs.

What will it cost

Line item budgets built from current US material and labor rates. 3 calculators.

Window Replacement Cost CalculatorPrice a whole-house window job: this window replacement cost calculator prices each opening by type, frame, glass and install method, insert against full frame.HVAC Cost CalculatorPrice an HVAC replacement: this HVAC cost calculator splits equipment from installation by system type, tonnage, efficiency tier, ductwork and gas or electrical work.Central Air Cost CalculatorThis central air cost calculator prices adding or replacing central air conditioning in 2026: equipment, labor, ductwork, electrical work, SEER2 tier and line set length.

A typical project, in order

Carry each result into the next tool.

1Insulation CalculatorBatts for walls and floors
2Blown-In Insulation CalculatorBags of cellulose or fiberglass for the attic
3Attic Ventilation CalculatorNet free area so the attic breathes
4Heat Loss CalculatorRoom heat loss once insulated
5BTU CalculatorCooling load for a room
6Furnace Size CalculatorHeating output for the whole house
7Mini Split CalculatorDuctless head size per zone

Insulation & HVAC questions

How many BTUs do I need per square foot?

About 20 BTU per square foot for cooling in a mild climate, 25 in a moderate one and 30 to 35 in a hot one. Heating runs 30 to 60 BTU per square foot depending on the climate zone.

What R-value should my attic have?

R-38 to R-60 for most of the US under DOE guidance. Blown cellulose at R-3.5 per inch needs about 11 inches for R-38.

Is a bigger furnace or AC better?

No. Oversized equipment short-cycles, wears faster and controls humidity poorly. Size to the load with a margin of 10 to 20%, not double.