The quick sizing table (square feet × climate)
Use this for a ballpark and for sanity-checking a contractor's quote — nothing more. It multiplies floor area by a BTU-per-square-foot factor that rises as the climate gets colder. The factors are derived from the same published envelope factors the calculator uses applied at each climate's typical design temperature — not from retail furnace charts, which is why they run well below the 25–40 BTU per square foot you'll see on many sales sites. Real homes vary widely around these numbers because insulation, air-tightness, ceiling height, and window area all move the load. Where a range is shown, the low end assumes a code-built or weatherized house and the high end an older, leakier one; a tight 2015+ build can fall below the low end, and a pre-1960 house with no upgrades can exceed the high end.
| Conditioned area | Mild ~5–10 BTU/sq ft |
Moderate ~10–18 BTU/sq ft |
Cold ~15–25 BTU/sq ft |
|---|---|---|---|
| 1,000 sq ft | 5,000–10,000 BTU/h ~0.5–1 ton | 10,000–18,000 BTU/h ~1–1.5 tons | 15,000–25,000 BTU/h ~1.5–2 tons |
| 1,500 sq ft | 7,500–15,000 BTU/h ~0.5–1.5 tons | 15,000–27,000 BTU/h ~1.5–2.5 tons | 22,500–37,500 BTU/h ~2–3 tons |
| 2,000 sq ft | 10,000–20,000 BTU/h ~1–1.5 tons | 20,000–36,000 BTU/h ~1.5–3 tons | 30,000–50,000 BTU/h ~2.5–4 tons |
| 2,500 sq ft | 12,500–25,000 BTU/h ~1–2 tons | 25,000–45,000 BTU/h ~2–3.5 tons | 37,500–62,500 BTU/h ~3–5 tons |
| 3,000 sq ft | 15,000–30,000 BTU/h ~1–2.5 tons | 30,000–54,000 BTU/h ~2.5–4.5 tons | 45,000–75,000 BTU/h ~4–6 tons |
Rule-of-thumb factors only. One ton = 12,000 BTU/h. "Mild" ≈ design temperature above ~30°F (much of the southern US); "moderate" ≈ ~10–25°F; "cold" ≈ below ~10°F (northern US and mountain states). These figures estimate the heating load; in hot southern climates the cooling load, not heating, often sets the size. They are for whole-home load estimation, not equipment selection.
The three numbers to gather before you shop
You don't need to be an HVAC pro to walk into a sizing conversation informed. Get these three things and you're most of the way there:
- Your conditioned floor area. Heated living space only — exclude an unheated basement, garage, or attic. This is the single biggest driver of load.
- Your local 99% heating design temperature. This is the outdoor temperature your area stays above 99% of the year — the cold your system has to handle without giving up. It's a real published number for your county, and it's what you size a cold-climate heat pump against. The calculator looks it up for you from an EPA reference by county.
- Roughly how tight and well-insulated the house is. A 2015-or-newer code build, a leaky pre-1960 house, and everything in between carry very different loads at the same square footage. You don't need an exact number — a rough era-and-condition category is enough to shift the estimate meaningfully.
Two optional extras make any estimate far more trustworthy: your existing furnace's output BTU/h from its nameplate, and how much fuel you burned last winter. Measured beats estimated every time, and the calculator can use either as a cross-check.
Now get your real number
The quick table above is a starting point. The free heat pump sizing calculator takes the three numbers you just gathered and does what a proper sizing conversation does: it estimates your design-day heating load as an honest range (not a single false-precision figure), then lets you check any specific unit's certified capacity at your design temperature — because a heat pump's heating output falls as it gets colder. ENERGY STAR certification requires third-party verified low-temperature performance, tested down to 5°F, and the calculator uses those certified rating points.
Open the free sizing calculator →
Want the long version — how design temperature, insulation tiers, and capacity-at-temperature all fit together? Read the full sizing guide and calculator, and see every constant and source on the methodology page.
Why right-sizing matters (both directions)
It's tempting to think bigger is safer. It isn't. ENERGY STAR is direct about it: a conventional system that's too large "may not keep your home comfortable because of frequent on/off cycling" — cycling that causes humidity control problems and inefficient operation — and oversized units "tend to have shorter lives due to 'short-cycle,' or turning on and off rapidly, which inflicts excessive wear and tear on the compressor." Oversizing also costs more up front and can run less efficiently. An undersized unit, meanwhile, can't keep up on the coldest days and leans on backup heat. Right-sizing — capacity that covers your real design load with only a small margin — is the target, and it's exactly what a Manual J calculation is for.
Common questions
What size heat pump do I need for a 2,000 square foot house?
As a rough starting estimate, a 2,000 sq ft home has a design-day heating load somewhere around 30,000–50,000 BTU/h (roughly 2.5–4 tons) in a cold climate and closer to 10,000–20,000 BTU/h (about 1–1.5 tons) where winters are mild. That is only a starting number. The real answer depends on your insulation, air-tightness, windows, and local design temperature — and a heat pump's rated "tons" shrink as it gets colder. ENERGY STAR's guidance is that your contractor verifies the size with a Manual J load calculation rather than a square-foot rule of thumb.
Is it better to oversize or undersize a heat pump?
Neither: you want it right-sized. ENERGY STAR notes that a system that is too large may not keep your home comfortable because of frequent on/off cycling, that oversized units tend to have shorter lives due to short-cycling, and that the same cycling causes humidity control problems. The Department of Energy's building-science guidance warns that rules of thumb too often produce excessively oversized systems. An undersized unit can't keep up on the coldest days. The goal is capacity that covers your real design-day load with only a small margin.
How many tons of heat pump per square foot?
The often-quoted rule of thumb is one ton (12,000 BTU/h) per 400–600 square feet, but that figure comes from air-conditioning practice and tends to oversize heat pump heating. Honest heating-load numbers run closer to one ton per 500–800 square feet in a cold climate and one ton per 1,000-plus square feet where winters are mild. Use per-square-foot math only to sanity-check a quote, never to make the final decision.
Does a heat pump lose capacity in cold weather?
Yes. A heat pump's heating output falls as the outdoor temperature drops, so a unit rated as "3 tons" at 47°F can deliver noticeably less when it's near or below 0°F. That is why you size a cold-climate unit by its certified capacity at your local design temperature, not by its nameplate tons. ENERGY STAR certification requires third-party verified low-temperature performance, with air-source heat pumps tested down to 5°F.
Related
- Cold-climate heat pump sizing calculator — the full tool: load range plus capacity-at-your-design-temperature checks and a certified-model finder.
- Methodology & sources — every constant, factor, and reference the calculator uses.