SizingKitHeating · equipment selection
Furnace size calculator — from a heating load to a nameplate
Free with no signup: give it a design heating load and it returns the input rating to shop for, because a furnace is sold on the fuel it burns and delivers that times its AFUE — so an 80% appliance has to be a quarter larger than the load and a 96% one barely larger at all. It applies the altitude derate above 2,000 ft, prints the 100 to 140% window the sizing standard allows, checks a nameplate you are considering against it, and works out the temperature rise the blower has to land inside. It does not calculate a load; four sibling pages do that.
- 100% free
- No signup
- Input vs output rating
- Altitude derate above 2,000 ft
- 100–140% sizing window
- Rise and fuel per hour
The load, and the three things that stand between it and a model number
This page does not compute a load. Bring one, and it turns it into a rating you can shop for.
Heat the building loses at the design condition — the output an appliance has to replace.
Default 80% — US federal minimum efficiency for non-weatherized gas furnaces, 80% AFUE, under the Department of Energy's appliance standards at 10 CFR 430.
No derate below 2,000 ft under the rule as written.
Default 4% — the appliance’s own installation instructions replace it.
Furnaces are sold in steps — 40, 60, 80, 100 MBH — so the arithmetic answer is rarely purchasable.
The rating
Output to replace
48,000 BTU/h
14.1 kW · 48 MBH
Input rating needed
60,000 BTU/h
Minimum, before any sizing margin
Input rating window
60 MBH – 84 MBH
60,000 BTU/h to 84,000 BTU/h
The nameplate you entered. Delivers 100% of the load, inside the 100% to 140% span the sizing table quotes for heating. Efficiency is what decides this one: at 80% AFUE the appliance throws away 12,000 BTU/h of the 60,000 BTU/h it burns.
Temperature rise across the heat exchanger
The one commissioning figure that is decided by the appliance and the duct system together, and the one an installer has to land inside.
The rise at that airflow
Rise at that airflow
37.0 °F / 20.6 °C
Enter the band stamped on the rating plate and this will check the rise against it. The band is the appliance manufacturer's and is not published as a standard, so it is not prefilled here.
Two altitude figures, and they disagree. At 0 ft the code rule as entered removes 0% of the input rating, while the ratio of atmospheric pressures from the ASHRAE barometric equation is 1, which is a 0% reduction. They are answering slightly different questions — one is a code allowance applied in whole thousands of feet, the other is the physics of thinner air — and the appliance’s listing settles it. The altitude derate the North American fuel gas codes apply to appliance input rating — NFPA 54 / ANSI Z223.1 in the United States, CSA B149.1 in Canada. This site does not reproduce either code; the figure is an editable default and the appliance's installation instructions and the authority having jurisdiction govern.
Fuel at full fire: 58.25 ft³ of natural gas per hour to deliver 48,000 BTU/h. Varies with composition — utility gas runs roughly 950 to 1150 BTU/ft³ and your utility publishes its own figure. A therm is 100,000 BTU by definition, so a ccf (100 ft³) is 1.03 therms at this heating value and not at any other. Averages. Natural gas heating value is set by the composition your utility delivers and changes through the year; propane and heating oil vary with temperature and blend. Where a bill states a heating value, that figure supersedes these. All four are higher heating values, which is what US utilities bill against and what AFUE is measured against — using an efficiency measured on a lower heating value with a higher heating value fuel figure overstates the result by about 10% on natural gas.
Why the ceiling exists. Heating output against the design heating load. Far more generous than the cooling limits because there is no humidity penalty on the heating side — only cycling losses. A furnace at twice its load reaches the setpoint in short bursts: each one begins with a pre-purge that throws warm air up the flue and ends with a post-purge that does it again, the blower never reaches steady state, and the rooms furthest from the plant never get their share before the burner stops. The house is warm at the thermostat and cold at the far end — which is read as an undersized furnace, and is the opposite. Two service details follow from the appliance rather than the load: a condensing furnace makes acidic condensate that needs a drain falling continuously to a trap, at the slope the plumbing code sets for that pipe, and a boiler replacing a furnace has a system water content that decides how badly it cycles, which comes out of the volume of the pipework it is attached to.
Where the load comes from. Not from here. Get it from a fabric and ventilation calculation if you have U-values and areas, from a room-by-room schedule if you need the cooling side too, from a certified Manual J if a permit is involved, and from the square-foot band only as a check that the other three are not wildly out.
How to turn a heating load into a furnace you can order
Load, then efficiency, then elevation — and only then a model.
Bring a design heating load, in any unit
BTU/h, MBH, kW or tons all read, and MBH is a thousand BTU/h because the M is the Roman numeral rather than the metric prefix. This is the output figure: what the building loses at the design condition and what the appliance has to put back.
Set the efficiency and the elevation before you look at any equipment
Those two are what separate the load from the rating on the box, and they move it in opposite directions — a higher AFUE means a smaller nameplate, a higher elevation means a larger one. At 6,000 ft the rule as written removes 16% of an appliance's input rating, so the nameplate has to be about 19% larger than the same load asks for at sea level.
Check a real nameplate against the window rather than the arithmetic
Furnaces come in steps of about 20 MBH, so the exact answer is almost never purchasable and the question becomes which of the two adjacent sizes to take. Enter the candidate and the page reports what it actually delivers as a percentage of the load, whether that falls inside the allowed span, and — with the band off the rating plate — whether your airflow puts the temperature rise where it belongs.
Technical specifications
| Ratings accepted | BTU/h, MBH, kW and tons in every field. A 60,000 BTU/h input at 96% AFUE and a 72,000 at 80% both deliver 57,600 BTU/h into the house — the same heat from two different numbers on the box, which is the whole reason the two ratings are kept apart. |
|---|---|
| Efficiency default | 80% AFUE, the current US federal minimum for a non-weatherized gas furnace. The 2023 DOE rule raises that class to 95% for units manufactured from late 2028, and several states and provinces already require condensing equipment. |
| Altitude derate | 4% of input rating per 1,000 ft above 2,000 ft as an editable default, from the North American fuel gas codes. The page also prints the actual atmospheric pressure ratio from the ASHRAE barometric equation, which at 5,000 ft is 0.832 against the rule's 0.88 — a real disagreement, settled by the appliance listing and not by this page. |
| Sizing window | 100% to 140% of the design heating load, from the equipment selection standard. Far wider than the cooling bounds because there is no humidity penalty on the heating side, only cycling losses. |
| Temperature rise | Output ÷ (1.08 × CFM), reported in °F and °C as a difference. The permitted band is stamped on the appliance and is not published as a standard, so the two band fields start empty and the check appears only once you fill them. |
| Fuel consumption | Natural gas at 1,030 BTU/ft³, propane at 91,500 BTU/gallon, No. 2 heating oil at 138,500 BTU/gallon, electricity at 3,412.142 BTU/kWh. The first three are averages your supplier's own figure supersedes; the fourth is an exact unit conversion. |
| Higher heating value | All four fuel figures are higher heating values, which is what US utilities bill against and what AFUE is measured against. Pairing a lower-heating-value efficiency with these overstates the result by about 10% on natural gas. |
| What it will not do | Compute a heating load, or tell you an appliance is safe. It reports what the ratings and the sizing table say and leaves the design signature to whoever holds one. |
Frequently asked questions
Is the number on the furnace the input or the output?
Almost always the input, and that is the trap this page exists for. A furnace advertised as 100,000 BTU is burning that much fuel; at 80% AFUE it delivers 80,000 into the house and sends the rest up the flue. Some nameplates state both and label them, and the output figure is the one to compare against a load — matching a 100,000 BTU/h load to a 100,000 BTU/h appliance leaves you 20,000 short on the coldest night.
Why does a bigger furnace heat a house worse?
Because heat has to be delivered as well as produced, and delivery takes time. An oversized appliance satisfies the thermostat in a few minutes of burner run, and in those minutes the ductwork is still warming up, the blower is still ramping and the rooms at the ends of the longest branches have barely started to receive air. The thermostat is in the hall, so it stops the cycle while the far bedroom is still cold — repeatedly, all evening. Each of those cycles also wastes the pre- and post-purge, so the appliance runs below its rated efficiency as well.
How does elevation change what I need to buy?
Thinner air means less oxygen through the same orifice, so the appliance can burn less fuel and the input rating has to come down. The consequence for sizing is that the nameplate has to go up: at 7,000 ft the code rule as written removes 20% of the rating, so a load needing 80,000 BTU/h of input at sea level needs a 100,000 nameplate there. High-altitude conversion kits change the orifices and sometimes the pressure switch, and above about 10,000 ft not all equipment is listed at all.
What temperature rise should I be seeing?
Whatever band is stamped on the rating plate, and the plate is the only authority — bands differ between models and between stages of the same model. What the figure tells you is the relationship between fire and airflow: too high means not enough air for the fire and the limit switch will eventually cut it out, too low means too much air, cool-feeling supply and, on a condensing appliance, condensation in places not designed to drain. Measuring it is a supply and a return thermometer, taken out of sight of the heat exchanger so radiant heat does not read into the supply probe.
Two-stage or modulating — does that change the sizing?
It changes the penalty for getting it wrong, not the target. A modulating appliance can run at a fraction of its rated fire, so it spends most of a mild winter behaving like a much smaller furnace and the cycling losses largely disappear. What it cannot do is exceed its maximum, so the upper end of the sizing question is unchanged and the lower end still has to meet the design load. Sizing bounds written as a single ratio describe single-stage equipment best, and the standards say so.
My contractor is quoting the same size as the old furnace. Is that reasonable?
Only if the old one was right and the building has not changed, and both are worth testing. Replacement-in-kind reproduces whatever the previous sizing decision was, including its errors, and it does it for another twenty years. If windows, insulation or air sealing have been done since, the load is definitively lower now. The cheap test is the fuel meter: on a cold day, clock the gas meter while the furnace runs and compare the input you measure against the nameplate and against the load — a furnace that satisfies the house on a third of its capacity at design weather is telling you something.
Does any of this apply to a boiler?
The input-and-output arithmetic and the efficiency figure do, and the airflow half does not. A boiler moves heat into water rather than air, so instead of a temperature rise across a heat exchanger you have a flow rate and a design temperature drop across the emitters, and instead of duct volume you have system water content — which is what decides how short its cycles are when it is oversized. A low-water-content boiler on a small zone can short-cycle in a way an old cast-iron one never did.
Input, output, and the two ratings a buyer meets in the wrong order
Every complaint about furnace sizing traces back to one thing: appliances are sold on what they consume and buildings are described by what they need. A furnace’s input rating is a fuel rate — how much gas it can burn in an hour — and the useful figure, the output, is that multiplied by an efficiency that has ranged from about 78% to 98% across the equipment on sale in the last thirty years. Two furnaces with the same number on the box can differ by 20,000 BTU/h of delivered heat, and two with different numbers can deliver the same. The arithmetic is trivial and the confusion is not, because nothing in a showroom conversation makes it obvious which rating is being quoted.
The second half of the problem is that the answer is not purchasable. Residential furnaces come in steps of roughly 20 MBH — 40, 60, 80, 100 — and a load of 48,000 BTU/h at 96% efficiency wants a 50,000 input that nobody makes. The real question is therefore always which of two adjacent sizes to take, and the sizing standard answers it with a span rather than a target: between the load itself and 140% of it. That upper figure is not generosity, it is the point at which cycling losses and comfort start to outweigh the reserve, and the standard’s own text is careful to say the bounds move between editions. Where a house is exactly between two sizes and both fall inside the span, the smaller one is the better default: it runs longer, delivers more evenly, and is recoverable with a strip heater or a setback schedule in a way that an oversized one is not recoverable at all.
What the page will not do is produce the load. That refusal is deliberate and it is what distinguishes this page from the calculators that ask for a square footage and hand back a furnace size in one step — the load is an engineering result about a building, the rating is a shopping question about a product, and merging them is how a rule of thumb gets laundered into an equipment decision. Bring the number from a fabric and ventilation heat loss, from a room-by-room schedule, or from a certified calculation where a permit demands one. If all you have is a square-foot band, the honest thing this page can tell you is that the band is wider than the step between two furnace sizes, which means it cannot choose between them.
What happens to the ratings you type
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Load figures, nameplate ratings and elevations are read by the JavaScript on this page and by nothing else — there is no analytics event on an input here, so the equipment you are pricing is not observable to anybody.