Heating, cooling & plumbing calculators

Septic Tank Size Calculator

Enter bedrooms, occupants and floor area, pick your state's rule, and get the design flow and the smallest tank the rule allows.

Gas Pipe Size Calculator

List your gas appliances and the longest run from the meter, and get the pipe size for every branch and the main line.

BTU Calculator

Enter the room, garage or shop size and how you use it, then get the BTUs to cool or heat it and the air conditioner or heater size to buy.

AC Size Calculator

Enter your home's square footage, climate zone and age to get the AC size in tons and BTU, the nearest unit to buy and a room-by-room check.

Furnace Size Calculator

Enter the heated square footage, climate zone, insulation and furnace efficiency to get the output you need and the furnace input size to buy.

Mini Split Size Calculator

Add each room you want a head in, then get the BTU per zone, the head size to buy and the outdoor unit for a single or multi-zone system.

Baseboard Heater Calculator

Enter the room size and ceiling height to get the watts of baseboard heat, the heater lengths to buy and the current they draw.

Space Heater Calculator

Space heater watts = room square feet × BTU per sq ft for your zone ÷ 3.412, so a 12 × 14 ft bedroom in zone 4 needs about 2,350 watts; enter your room for watts, BTU and heater count.

Wood Stove Size Calculator

Wood stove BTU output = heated square feet × BTU per sq ft for your zone, so a 1,500 sq ft home in zone 4 needs about 71,000 BTU per hour, a stove with a 2 cu ft firebox; enter yours for output and models.

Heat Pump Size Calculator

Heat pump tons = square feet ÷ square feet per ton, so a 2,000 sq ft older home in a mixed climate at 500 sq ft per ton needs a 4 ton unit; enter yours for tons, BTU and backup heat.

Ductwork Calculator

Enter the system size and your rooms with their duct runs to get the CFM, trunk and branch sizes, velocities and a material list.

CFM Calculator

Pick what you are sizing: a room's air changes, a system's tons, a duct's velocity, a bathroom fan or a range hood, and get the CFM.

Duct Size Calculator

Enter the airflow and a friction rate or target velocity to get the round duct size, rectangular equivalents and the velocity and friction at that size.

Dehumidifier Size Calculator

Dehumidifier pints per day = the ENERGY STAR range for how damp the space is, so a 1,000 sq ft very damp basement needs about 30 pints, 35 below grade; enter your space for pints per day.

Bathroom Fan CFM Calculator

Bathroom fan CFM = 1 CFM per square foot of floor up to 100 sq ft with a 50 CFM minimum, so an 8 × 10 ft bath needs an 80 CFM fan; enter your bathroom for the size, or add fixtures above 100 sq ft.

Range Hood CFM Calculator

Range hood CFM = total gas burner BTU ÷ 100, so a 40,000 BTU gas range needs a 400 CFM hood; for electric ranges use 100 CFM per foot of width on a wall and 150 on an island, plus duct losses.

Firewood Calculator

A cord is 128 cu ft of stacked wood, so a stack 16 ft long, 4 ft high and 16 in deep is two thirds of a cord; enter your stack or your house to get cords, price per cord and cords per winter.

Propane Usage Calculator

Propane gallons = BTU per hour × hours ÷ 91,452 BTU per gallon, so a 100,000 BTU furnace running 6 hours burns 6.6 gallons; enter your home or appliances for gallons per month and tank refill timing.

Water Softener Size Calculator

Softener grains per day = people × gallons per person × (hardness + 5 × iron ppm); 4 people at 10 gpg use 3,200 grains a day and need a 32,000 grain softener.

Tankless Water Heater Size Calculator

Tankless Btu/h = GPM × temperature rise × 500 ÷ efficiency; two showers and a kitchen sink at a 65 °F rise need 6.5 GPM and about 222,000 Btu/h.

Well Pump Size Calculator

Well pump GPM = one gallon per minute per fixture; 12 fixtures at 150 ft to water and 50 psi need 12 GPM against 285 ft of head, a 1 1/2 hp pump.

Pipe Slope Calculator

Pipe drop = length × 1/4 inch per foot; a 50 ft drain falls 12.5 in, a 2.08% slope or 1.19 degrees, the minimum for pipe up to 2 1/2 in.

Water Pressure Calculator

Water pressure in psi = height in feet × 0.433; 100 ft of water pushes 43.3 psi, and 185 ft reaches the 80 psi code limit.

Garage Door Spring Calculator

Spring IPPT = door weight × drum radius ÷ turns; a 182 lb, 7 ft door on 4 in drums needs 47.4 IPPT, 23.7 per spring, such as .218 wire, 2 in ID, 24 3/4 in long.

Garage Door Size Calculator

A garage door needs an opening the same size as the door plus 12 in of headroom and 3 3/4 in of side room; a 16 × 7 ft door needs 192 × 84 in and 102 in of backroom.