IRC M1401.3 against the 30 to 60 BTU per square foot rule
A leading price guide tells homeowners to size a furnace at 30 to 60 BTU per square foot, which lets a 2,000 square foot house have anything from 60,000 to 120,000 BTU. The residential code does not accept floor area at all: section M1401.3 sizes heating equipment by ACCA Manual S from a Manual J load calculation. This page shows how wide the rule is for your house, what the code asks for instead, and what a replacement costs.
The floor area rule, the code, and the price
sq ft
Heated floor area only. This does not size a furnace; it shows what the floor area rule allows and prices the job.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
The source gives $2,823 to $6,896 for a replacement, $4,814 on average. It also prints a table of installed cost by house size at exactly $1.20 to $2.00 a square foot, which puts a 2,000 sq ft house at $2,400 to $4,000, below the low end of its own gas furnace row; that table is left out. Its headline reads 2025 on a page dated June 2026.
The source's headline starts at $1,250, its standard furnace row at $1,000, and its text at $380.
Worked example
A 2,000 square foot house replacing a gas furnace.
Rule of thumb. 2,000 × 30 to 60 = 60,000 to 120,000 BTU.
Width. The high end is twice the low; any furnace between passes the rule.
Code. M1401.3: sized by Manual S from a Manual J load.
Price. Gas furnace $3,800 to $10,000 installed, plus $60 to $330 to remove the old one.
The same guide's size table puts this house's furnace at $2,400 to $4,000 installed, below its own gas furnace row. The type table is the one used here.
The formula
The rule, for what it is worth:
BTU = floor area × 30 to 60
30 to 60
HomeAdvisor's range, June 2026
Manual J
the load calculation the IRC refers to
Manual S
the equipment selection method that follows it
A factor of two in the rule covers the whole difference between a mild, tight house and a cold, leaky one, which is what a load calculation is for.
Why this page does not give a furnace size
Furnace size calculators multiply floor area by a number. The code a furnace is installed under does not size equipment that way.
The load calculation it names, ACCA Manual J, is not published free, so this page shows the width of the rule, quotes the code, and prices the job from sourced figures.
A rule of thumb twice as wide as it is useful, and a code that does not allow one
The price guide sizes a furnace by floor area:
To figure out what size you need, estimate about 30 to 60 BTUs per square foot. So, for a 2,000-square-foot home, you'd need a furnace rated between 60,000 and 120,000 BTUs.
HomeAdvisor furnace size and installed cost by house size, updated June 17, 2026, against its own gas furnace row
House
Guide's BTU range
High against low
Guide's installed cost
Per sq ft
Guide's gas furnace row
1,000 sq ft
30,000 to 60,000 BTU
2 times
$1,200 to $2,000
$1.20 to $2.00
$3,800 to $10,000
2,000 sq ft
60,000 to 120,000 BTU
2 times
$2,400 to $4,000
$1.20 to $2.00
$3,800 to $10,000
3,000 sq ft
90,000 to 180,000 BTU
2 times
$3,600 to $6,000
$1.20 to $2.00
$3,800 to $10,000
4,000 sq ft
120,000 to 240,000 BTU
2 times
$4,800 to $8,000
$1.20 to $2.00
$3,800 to $10,000
5,000 sq ft
150,000 to 300,000 BTU
2 times
$6,000 to $10,000
$1.20 to $2.00
$3,800 to $10,000
Every row spans a factor of two in BTU. And every installed cost is exactly $1.20 to $2.00 a square foot, so a 2,000 sq ft house is priced at $2,400 to $4,000 while the same guide starts a gas furnace at $3,800; for houses up to 3,000 sq ft the low end is below that.
The residential code sizes equipment another way, in Virginia's adoption of the 2021 IRC:
Heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.
Utah's adoption goes further and says who may do the calculation:
An individual performing load calculations shall be qualified by completing HVAC training from one of the following: HVAC load calculation education from ACCA; A recognized educational institution; HVAC equipment manufacturer's training; or Other recognized industry certification.
This page does not size a furnace. ACCA's Manual J is not published free, and a floor area rule is not what the code asks for. The calculator shows the rule's range for your house so you can see how wide it is, and prices the job.
What to ask for with a quote
The Manual J report. It lists the design heating load in BTU per hour, from the house's walls, windows, insulation, air leakage and the local design temperature. A furnace's output should be close to it.
Output, not input. A furnace's rating plate gives input BTU; its output depends on efficiency. Compare the load with output capacity.
A replacement sized to the old furnace is not a load calculation either. Houses change: insulation, windows and air sealing added since the old one was installed all reduce the load.
Utah Residential Code 2021, Chapter 14 Heating and Cooling Equipment and Appliances, International Residential Code 2021 as adopted with amendments in Utah, read through UpCodes with the jurisdiction and edition stated on the page. Utah's amended M1401.3 and its added M1401.3.1, which requires HVAC training of whoever performs the load calculation.
How Much Does a Furnace Replacement Cost?, HomeAdvisor staff writer, reviewed by After Hours Home Improvement, LLC, updated June 17, 2026. The 30 to 60 BTU per square foot rule, the table by furnace type and the table by house size.
A load calculation needs the house, not its area: wall, ceiling and window areas and their insulation, air leakage, duct location, orientation and the local design temperature. Ask the installer for the Manual J report with the quote; it is the document the code refers to.
Frequently asked questions
What size furnace do I need for 2,000 square feet?
By HomeAdvisor's 30 to 60 BTU per square foot rule, anywhere from 60,000 to 120,000 BTU. The residential code, IRC M1401.3, requires sizing by ACCA Manual S from a Manual J load calculation instead, which narrows it to a figure for your house.
How many BTU per square foot for a furnace?
HomeAdvisor gives 30 to 60. The spread is a factor of two, which is why the code does not use floor area to size equipment.
Does the code require a Manual J?
IRC M1401.3 requires heating and cooling equipment to be sized by ACCA Manual S or other approved methods, based on loads from ACCA Manual J or other approved methods. Utah's adoption also requires the person doing the calculation to have HVAC load calculation training.
How much does a new furnace cost?
HomeAdvisor's June 2026 guide gives $3,800 to $10,000 for a gas furnace, $2,000 to $7,000 electric and $6,750 to $10,000 oil, installed, plus $60 to $330 to remove the old one.
Is a bigger furnace better?
IRC M1401.3 lets equipment exceed the Manual S capacity only in listed cases, such as multistage equipment whose range covers the load, a load no unit meets on both total and sensible heat where the next larger size is chosen, or where the specified unit is the manufacturer's smallest. Oversizing is not otherwise part of the method.
Can I size a furnace myself?
The rule of thumb is not a sizing method under the code. A Manual J calculation needs the house's construction details and, in Utah, someone with HVAC load calculation training.
Check these numbers yourself
Virginia Residential Code 2021, Chapter 14IRC 2021 as adopted in Virginia, read 2026-09-13 through UpCodes with the jurisdiction and edition stated on the page. Section M1401.3.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.
This page does not size a furnace. The code sections are quoted from Virginia's and Utah's 2021 adoptions, read on 13 September 2026; your jurisdiction may differ. Prices are HomeAdvisor's, not this site's. Gas and electrical work on a furnace is commonly permit work.