Furnace Size Chart: BTU by Home Size & Climate Zone

What does a 2,000 sq ft home in the cold zone needing 90,000–110,000 BTU mean?

On the default — 2,000 sq ft in the cold zone — the furnace needs 90,000–110,000 BTU of heat an hour, and the recommended furnace is 100,000 BTU. The range comes from the rate: the cold zone needs 45–55 BTU per sq ft, so 2,000 × 45 = 90,000 BTU and 2,000 × 55 = 110,000 BTU. The midpoint is 100,000 BTU, and the nearest standard furnace size to that midpoint is 100,000 BTU. Whether that is the right furnace depends on the house — a draughty house with tall ceilings needs the high end of the range, and a tight, well-insulated one needs the low end.

Three things about the model are worth stating plainly.

First, the range is 90,000–110,000 BTU and the furnace is 100,000 BTU — the range and the furnace are two outputs, never the same number. Second, the meaning is 100,000 BTU of furnace for 2,000 sq ft in the cold zone — the furnace that heats the home without short-cycling, not the heat load itself. And third, the furnace is a step function of the midpoint: 1,800 sq ft in the cold zone takes an 80,000 BTU furnace, but 1,801 sq ft takes a 100,000 BTU furnace — one square foot crosses the tie — all listed in Accuracy & limitations.

The full chart

800–1,200 sq ft: mild 20,000–42,000 BTU; moderate 28,000–54,000 BTU; cold 36,000–66,000 BTU; very cold 44,000–78,000 BTU — The smallest homes — 20,000–42,000 BTU in the mild zone, up to 44,000–78,000 BTU in the very cold zone.

1,200–1,600 sq ft: mild 30,000–56,000 BTU; moderate 42,000–72,000 BTU; cold 54,000–88,000 BTU; very cold 66,000–104,000 BTU — Small homes — 30,000–56,000 BTU in the mild zone, up to 66,000–104,000 BTU in the very cold zone.

1,600–2,000 sq ft: mild 40,000–70,000 BTU; moderate 56,000–90,000 BTU; cold 72,000–110,000 BTU; very cold 88,000–130,000 BTU — Mid-size homes — 40,000–70,000 BTU in the mild zone, up to 88,000–130,000 BTU in the very cold zone.

2,000–2,500 sq ft: mild 50,000–87,500 BTU; moderate 70,000–112,500 BTU; cold 90,000–137,500 BTU; very cold 110,000–162,500 BTU — The default row — 90,000–137,500 BTU in the cold zone, and a 100,000 BTU furnace for a 2,000 sq ft home.

2,500–3,000 sq ft: mild 62,500–105,000 BTU; moderate 87,500–135,000 BTU; cold 112,500–165,000 BTU; very cold 137,500–195,000 BTU — Larger homes — 62,500–105,000 BTU in the mild zone, up to 137,500–195,000 BTU in the very cold zone.

3,000–4,000 sq ft: mild 75,000–140,000 BTU; moderate 105,000–180,000 BTU; cold 135,000–220,000 BTU; very cold 165,000–260,000 BTU — The largest homes in the table — 75,000–140,000 BTU in the mild zone, up to 165,000–260,000 BTU in the very cold zone.

Adjustment notes — ceiling height, insulation, windows

Ceiling height. The chart assumes 8 ft ceilings. With 10 ft ceilings the heated volume is a quarter bigger, so use the high end of the range — or the next band up — and with vaulted ceilings ask an HVAC pro for a Manual J calculation.

Insulation. A well-insulated home with sealed ducts sits at the low end of the range; a poorly insulated one with draughty windows sits at the high end. Upgrading the insulation is usually cheaper than upgrading the furnace.

Windows. Large single-pane windows lose far more heat than a wall of the same size, so a home with a lot of glass needs the high end of the range — and new windows can shrink the furnace you need.

Air leakage. A tight home holds its heat; a leaky one does not. If the house has never been air-sealed, use the high end of the range.

The worked example, line by line

Setup: home size 2,000 sq ft, climate zone cold.

Line 1 — the rate. The cold zone needs 45–55 BTU per sq ft.

Line 2 — the low end. 2,000 × 45 = 90,000 BTU.

Line 3 — the high end. 2,000 × 55 = 110,000 BTU.

Line 4 — the midpoint. (90,000 + 110,000) ÷ 2 = 100,000 BTU.

Line 5 — the furnace. The nearest standard size to 100,000 BTU is 100,000 BTU.

Line 6 — the check. At 1,800 sq ft the furnace is 80,000 BTU — the midpoint is exactly 90,000 BTU, tied between 80,000 and 100,000, so the smaller size wins; and at 1,801 sq ft it is 100,000 BTU.

Every branch, checked

1,000 sq ft, mild zone: 1,000 sq ft, mild zone: 25,000–35,000 BTU — 40,000 BTU furnace

1,500 sq ft, moderate zone: 1,500 sq ft, moderate zone: 52,500–67,500 BTU — 60,000 BTU furnace

1,800 sq ft, moderate zone: 1,800 sq ft, moderate zone: 63,000–81,000 BTU — 80,000 BTU furnace

2,000 sq ft, cold zone: 2,000 sq ft, cold zone: 90,000–110,000 BTU — 100,000 BTU furnace

2,500 sq ft, mild zone: 2,500 sq ft, mild zone: 62,500–87,500 BTU — 80,000 BTU furnace

3,000 sq ft, cold zone: 3,000 sq ft, cold zone: 135,000–165,000 BTU — 140,000 BTU furnace

3,500 sq ft, moderate zone: 3,500 sq ft, moderate zone: 122,500–157,500 BTU — 140,000 BTU furnace

1,200 sq ft, cold zone: 1,200 sq ft, cold zone: 54,000–66,000 BTU — 60,000 BTU furnace

The reverses, worked backwards

Smallest home where the cold-zone high reaches 110,000 BTU: At the displayed 2,000 sq ft the figures are 2,000 sq ft, cold zone: 90,000–110,000 BTU — 100,000 BTU furnace — midpoint 100,000 BTU. At 1,999 sq ft the figures are 1,999 sq ft, cold zone: 89,955–109,945 BTU — 100,000 BTU furnace — midpoint 99,950 BTU — the high is 109,945 BTU — so 2,000 sq ft is the smallest home where the cold-zone high reaches 110,000 BTU.

Smallest home where the moderate-zone furnace reaches 80,000 BTU: At the displayed 1,751 sq ft the figures are 1,751 sq ft, moderate zone: 61,285–78,795 BTU — 80,000 BTU furnace — midpoint 70,040 BTU. At 1,750 sq ft the figures are 1,750 sq ft, moderate zone: 61,250–78,750 BTU — 60,000 BTU furnace — midpoint 70,000 BTU — the midpoint is exactly 70,000 BTU, tied, so the furnace is 60,000 BTU — so 1,751 sq ft is the smallest home where the moderate-zone furnace reaches 80,000 BTU.

Smallest home where the mild-zone low reaches 50,000 BTU: At the displayed 2,000 sq ft the figures are 2,000 sq ft, mild zone: 50,000–70,000 BTU — 60,000 BTU furnace — midpoint 60,000 BTU. At 1,999 sq ft the figures are 1,999 sq ft, mild zone: 49,975–69,965 BTU — 60,000 BTU furnace — midpoint 59,970 BTU — the low is 49,975 BTU — so 2,000 sq ft is the smallest home where the mild-zone low reaches 50,000 BTU.

Smallest home where the very-cold-zone furnace reaches 120,000 BTU: At the displayed 1,834 sq ft the figures are 1,834 sq ft, very cold zone: 100,870–119,210 BTU — 120,000 BTU furnace — midpoint 110,040 BTU. At 1,833 sq ft the figures are 1,833 sq ft, very cold zone: 100,815–119,145 BTU — 100,000 BTU furnace — midpoint 109,980 BTU — the midpoint is 109,980 BTU — a 100,000 BTU furnace — so 1,834 sq ft is the smallest home where the very-cold-zone furnace reaches 120,000 BTU.

Smallest home where the cold-zone furnace reaches 100,000 BTU: At the displayed 1,801 sq ft the figures are 1,801 sq ft, cold zone: 81,045–99,055 BTU — 100,000 BTU furnace — midpoint 90,050 BTU. At 1,800 sq ft the figures are 1,800 sq ft, cold zone: 81,000–99,000 BTU — 80,000 BTU furnace — midpoint 90,000 BTU — the midpoint is exactly 90,000 BTU, tied, so the furnace is 80,000 BTU — so 1,801 sq ft is the smallest home where the cold-zone furnace reaches 100,000 BTU.

Smallest home where the mild-zone high reaches 100,000 BTU: At the displayed 2,858 sq ft the figures are 2,858 sq ft, mild zone: 71,450–100,030 BTU — 80,000 BTU furnace — midpoint 85,740 BTU. At 2,857 sq ft the figures are 2,857 sq ft, mild zone: 71,425–99,995 BTU — 80,000 BTU furnace — midpoint 85,710 BTU — the high is 99,995 BTU — so 2,858 sq ft is the smallest home where the mild-zone high reaches 100,000 BTU.

A warning about the neighbours: every neighbour above sits one square foot from the star, and every neighbour figure is a different furnace even though the home barely changed — the tie, not the exact size, decides. The page prints the full chart so the rounding does not mislead you: R1 2,000 / 1,999 sq ft; R2 1,751 / 1,750 sq ft; R3 2,000 / 1,999 sq ft; R4 1,834 / 1,833 sq ft; R5 1,801 / 1,800 sq ft; R6 2,858 / 2,857 sq ft.

What the defaults assume

The defaults are a 2,000 sq ft home in the cold zone — 2,000 sq ft, cold zone: 90,000–110,000 BTU — 100,000 BTU furnace. With 1,000 sq ft, mild zone, 1,000 sq ft, mild zone: 25,000–35,000 BTU — 40,000 BTU furnace. With 1,500 sq ft, moderate zone, 1,500 sq ft, moderate zone: 52,500–67,500 BTU — 60,000 BTU furnace. With 1,800 sq ft, moderate zone, 1,800 sq ft, moderate zone: 63,000–81,000 BTU — 80,000 BTU furnace. With 2,000 sq ft, cold zone, 2,000 sq ft, cold zone: 90,000–110,000 BTU — 100,000 BTU furnace. With 2,500 sq ft, mild zone, 2,500 sq ft, mild zone: 62,500–87,500 BTU — 80,000 BTU furnace. With 3,000 sq ft, cold zone, 3,000 sq ft, cold zone: 135,000–165,000 BTU — 140,000 BTU furnace. With 3,500 sq ft, moderate zone, 3,500 sq ft, moderate zone: 122,500–157,500 BTU — 140,000 BTU furnace. With 1,200 sq ft, cold zone, 1,200 sq ft, cold zone: 54,000–66,000 BTU — 60,000 BTU furnace. With 0 sq ft, Enter your home size in square feet. — no home to heat — the calculator says so plainly. With 500 sq ft, mild zone, 500 sq ft, mild zone: 12,500–17,500 BTU — 40,000 BTU furnace — midpoint 15,000 BTU — the smallest standard furnace — 40,000 BTU. With 5,000 sq ft, cold zone, 5,000 sq ft, cold zone: 225,000–275,000 BTU — 200,000 BTU furnace — midpoint 250,000 BTU — above the table — the table tops out at the 200,000 BTU furnace. With 1,750.5 sq ft, moderate zone, 1,750.50 sq ft, moderate zone: 61,267.50–78,772.50 BTU — 80,000 BTU furnace — midpoint 70,020 BTU — a fractional size — the range and the furnace both follow. With 800 sq ft, very cold zone, 800 sq ft, very cold zone: 44,000–52,000 BTU — 40,000 BTU furnace — midpoint 48,000 BTU — a small home in the coldest zone — 40,000 BTU. With 1,200 sq ft, moderate zone, 1,200 sq ft, moderate zone: 42,000–54,000 BTU — 40,000 BTU furnace — midpoint 48,000 BTU — a small home in a mild zone — 40,000 BTU. With 4,000 sq ft, mild zone, 4,000 sq ft, mild zone: 100,000–140,000 BTU — 120,000 BTU furnace — midpoint 120,000 BTU — a big home in a mild zone — 120,000 BTU. With 100 sq ft, mild zone, 100 sq ft, mild zone: 2,500–3,500 BTU — 40,000 BTU furnace — midpoint 3,000 BTU — the smallest home there is — 40,000 BTU. With 2,500 sq ft, very cold zone, 2,500 sq ft, very cold zone: 137,500–162,500 BTU — 140,000 BTU furnace — midpoint 150,000 BTU — the midpoint is exactly 150,000 BTU, tied, so the smaller size wins. With 2,200 sq ft, moderate zone, 2,200 sq ft, moderate zone: 77,000–99,000 BTU — 80,000 BTU furnace — midpoint 88,000 BTU — a mid-size home — 80,000 BTU. With 10,000 sq ft, cold zone, 10,000 sq ft, cold zone: 450,000–550,000 BTU — 200,000 BTU furnace — midpoint 500,000 BTU — far above the table — the table tops out at the 200,000 BTU furnace. With 1,500 sq ft, cold zone, 1,500 sq ft, cold zone: 67,500–82,500 BTU — 80,000 BTU furnace — midpoint 75,000 BTU — a mid-size home in the cold zone — 80,000 BTU.

A note on the rounding: the BTU figures are whole numbers, and the midpoint is the average of the two ends. With a fractional home size the low and high can carry halves — 1,750.5 sq ft in the moderate zone gives 61,267.50–78,772.50 BTU — and the page prints the halves where they occur so the rounding does not mislead you.

Output · formula · meaning

Who uses this

• The homeowner replacing a furnace: they learn a 2,000 sq ft home in the cold zone needs a 100,000 BTU furnace — and they check the 1,800 sq ft branch to see how close they are to the next size down.
• The buyer checking a quote: they learn the range for their home and zone — and they check whether the quoted furnace sits at the midpoint or oversized.
• The DIYer planning a new build: they learn the BTU per sq ft for each zone — and they check the adjustment notes to see whether their ceilings and windows push them to the high end.
• The HVAC student: they learn how the chart and the calculator agree — and they check the reverses to see where each furnace size steps up.

Accuracy & limitations

The calculator does the arithmetic exactly — home size times the rate for the zone, midpoint, nearest standard furnace size with ties to the smaller — and every figure on this page is checked against an independent model. The rates are the same ones the furnace-size calculator on this site uses, widened into a range, so the chart and the calculator agree. What it cannot do is a Manual J load calculation: ceiling height, insulation, windows, air leakage, orientation and duct losses all move the real load, and a home at the edge of a band can need the next size up or down. A furnace that is too big short-cycles — it heats the house in bursts, wears out faster and leaves cold spots — so when the midpoint ties, the chart picks the smaller size. For a final purchase, get a Manual J from an HVAC pro.

Cite this calculator

BTU per sq ft: mild 25–35, moderate 35–45, cold 45–55, very cold 55–65. On the defaults — 2,000 sq ft, cold zone — the furnace needs 90,000–110,000 BTU and the recommended furnace is 100,000 BTU. Smallest home where the cold-zone high reaches 110,000 BTU: 2,000 sq ft. Smallest home where the moderate-zone furnace reaches 80,000 BTU: 1,751 sq ft. Smallest home where the mild-zone low reaches 50,000 BTU: 2,000 sq ft. Smallest home where the very-cold-zone furnace reaches 120,000 BTU: 1,834 sq ft. Smallest home where the cold-zone furnace reaches 100,000 BTU: 1,801 sq ft. Smallest home where the mild-zone high reaches 100,000 BTU: 2,858 sq ft. Source: Bright Side Kit, brightoolbox.com.

Frequently asked questions