Calculator
HVAC Refrigeration BTU Calculator
Size a walk-in cooler or freezer on site, enter box dimensions, holding and ambient temperature, panel insulation, daily product load, and door usage to get the required condensing-unit capacity in BTU/hr and tons, with every load component itemised. Free, no signup.
Free
Product loadHeat pulled out of everything you put in the box each day, plus respiration heat for fruit and vegetables, which is charged on total pounds stored.
Lights, motors, people & doorsMiscellaneous internal load. The method averages each source over a full 24 hours so the equipment has capacity while they run.
Envelope & design assumptionsFloor construction, door usage, sun on the roof, safety factor, and condensing-unit run time. The published defaults are already loaded.
Required equipment capacity
7,104 BTU/hr
0.59 tons at 16 hr run time · cooler
4″ meets the 3″ standard for a 35°F room
Total load
113,666 BTU/24hr103,332 BTU/24hr + 10% safety factor
Tons of refrigeration
0.59 tons1 ton = 12,000 BTU/hr
Box volume
960 ft³120 sq ft floor · 352 sq ft of wall
Infiltration share
41.6%High, strip curtains or an air curtain will pay back
Load breakdown (BTU/hr)
- Walls, ceiling & floor2,212 BTU/hr
- Air change / infiltration2,689 BTU/hr
- Product pull-down361 BTU/hr
- Lights, motors & people1,196 BTU/hr
- Safety factor (10%)646 BTU/hr
- Total7,104 BTU/hr
Reference tables used
- Wall & ceiling R-value
- R-33.3 · 4″ polyurethane, foamed in place
- Floor R-value
- R-4.8 · 6″ concrete slab, uninsulated
- Air changes per 24 hr
- 18.00 (above 32°F table)
- Heat removed per ft³ of air
- 2.49 BTU/ft³ at 95°F / 50% RH
- Product specific heat
- 0.77 above / 0.40 below freezing
- Latent heat of fusion
- 100 BTU/lb · freezes at 28°F
- Respiration rate
- 0.00 BTU/lb/24 hr
- Occupancy heat
- 21,480 BTU/person/24 hr
Wall heat loads, air changes, infiltration enthalpy, product properties, and occupancy heat all come from the ASHRAE Handbook of Fundamentals tables reprinted in the Heatcraft Engineering Manual load-estimate forms.
Industry benchmarks
- Cooler run time (no timer)
- 16 hr
- Freezer run time (electric defrost)
- 18 hr
- Safety factor on total load
- 10%
- Lighting allowance
- 1-1.5 W/sq ft
- Motor heat
- 75,000 BTU/HP/24 hr
- Motors, if unknown
- 1 HP / 16,000 ft³ cooler
- People, if unknown
- 1 person / 25,000 ft³
- Glass reach-in door
- 19,200 (cooler) / 31,200 (freezer) BTU/day
- Freezing point, if unknown
- 28°F
- 1 ton of refrigeration
- 12,000 BTU/hr
What you'll get
Describe the box
Pick walk-in cooler, walk-in freezer, prep room, or blast freezer to load the standard holding temperature, panel spec, and compressor run time, then enter your actual length, width, height, and design ambient.
Add product and internal load
Choose the stored product to pull its specific heat, latent heat of fusion, and freezing point automatically, then add daily pounds, pull-down hours, lighting, evaporator motors, people, and glass reach-in doors.
Get equipment capacity
See required BTU/hr and tons after the 10% safety factor and run-time division, plus a breakdown showing how much of the load is walls, infiltration, product, and internal heat.
How it works
- 1
Describe the box
Pick walk-in cooler, walk-in freezer, prep room, or blast freezer to load the standard holding temperature, panel spec, and compressor run time, then enter your actual length, width, height, and design ambient.
- 2
Add product and internal load
Choose the stored product to pull its specific heat, latent heat of fusion, and freezing point automatically, then add daily pounds, pull-down hours, lighting, evaporator motors, people, and glass reach-in doors.
- 3
Get equipment capacity
See required BTU/hr and tons after the 10% safety factor and run-time division, plus a breakdown showing how much of the load is walls, infiltration, product, and internal heat.
Why walk-in refrigeration is sized differently from comfort cooling
A walk-in is not a small air conditioner. Comfort cooling sizes to a peak hourly gain; refrigeration sizes a 24-hour heat load and then divides it by how many hours the condensing unit is allowed to run, 16 hours on a 35°F cooler with no timer, 18 on a freezer with electric defrost. Skip that division and you undersize the unit by a third. This calculator runs the full four-part box load: transmission through walls, ceiling, and floor; the air-change load from door openings; product pull-down including the latent heat of freezing and respiration heat from produce; and internal load from lights, evaporator motors, people, and glass doors.
The result is where most walk-in problems get diagnosed before the equipment is ordered. On a small box, infiltration is routinely 40% of the load, which is why strip curtains and door closers pay for themselves faster than a bigger compressor. An uninsulated slab under a cooler often outweighs all four walls combined. And a freezer floor is calculated against ground temperature, not ambient. For residential and light-commercial comfort sizing use the BTU Calculator or the HVAC Load Calculator instead, those are Manual J-style room and whole-building tools and will not size refrigeration.
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Refrigeration BTU Calculator FAQs
There is no reliable per-square-foot rule, because the load scales with surface area, door usage, and how much warm product goes in each day. A 10×12 restaurant walk-in cooler on average door usage typically lands around 7,000-9,000 BTU/hr, but the same box as a freezer, or with heavy door traffic and a daily product delivery, can be double that. Enter your actual box and load rather than using a multiplier.
Refrigeration equipment is not sized to run continuously, it needs off time for defrost and for the coil to drain. The industry load forms total the heat gain over 24 hours and then divide by the intended compressor run time: 16 hours for a 35°F cooler with no timer, 18 hours for a storage freezer or blast freezer on electric defrost, and 20-22 hours for a 50°F room with a coil temperature above freezing.
Every time the door opens, warm humid air walks in and has to be cooled and dehumidified. The load is the box volume times the air changes per 24 hours for that volume, times the heat removed per cubic foot at your room and ambient conditions. On a small walk-in it is often 30-45% of the total load, and doubling for heavy door usage is standard, which is why door closers, strip curtains, and air curtains change the equipment size.
Those two size comfort heating and cooling for occupied rooms and homes using Manual J-style methods. This one sizes refrigeration for a walk-in box: it works in BTU per 24 hours, models product pull-down including latent heat of fusion and produce respiration, applies a 10% safety factor, and divides by compressor run time to give the condensing-unit capacity you actually order.
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