How much weight can a deck hold?

A deck designed to the IRC base carries 40 psf live plus 10 psf dead, so a 16x12 deck is designed for about 9,600 lb spread evenly. That is a design load, not a test result: it says what the deck was sized for, not what it will fail at.

Is deck live load always 40 psf?

No. 40 psf is the IRC base figure in Table R301.5. Washington State amended it to 60 psf for decks and balconies, and other jurisdictions may too. Ask your building department which figure applies before sizing anything.

If my jurisdiction requires 60 psf, can I still use the IRC span tables?

No. The Washington State Building Code Council was asked exactly this and answered that the amendment does not include amended tables, so it published replacements computed at 60 psf. Joist spans fell about 11% and ledger bolt spacing tightened about 38%.

What is the difference between live load and dead load?

Dead load is the deck itself: boards, joists, beam, railing. Live load is everything you put on it, people and furniture. The IRC assumes 10 psf dead for a typical wood deck and sets the live load separately.

How much load goes on each deck footing?

On a ledger-hung deck the house takes about half and the beam takes the other half, so each footing carries roughly the deck area times the design load, divided by twice the number of posts. On a 16x12 at 50 psf with three posts that is about 1,600 lb each.

What soil bearing capacity should I use?

Whatever your building department or soils report gives you. It is a site property, not a lookup, and this page leaves the field at zero rather than guessing. Presumptive values in the code exist but vary by soil class and jurisdiction.

Does a hot tub change the calculation?

Completely, and this page does not handle it. A filled hot tub is a concentrated load far above 40 psf over a small area and needs its own framing design, usually an engineer's.

Home › Decking › Deck Load Calculator

Deck Load Calculator

Live load is an input, because it is not 40 psf everywhere

Most deck load calculators hard-code 40 psf and hand you a number. The IRC base is 40 psf live plus 10 dead, but at least one state amends it to 60, and the consequence is not what people assume: the joist, beam and ledger tables printed in the code were computed at 40 and do not scale. This traces the load down the deck and says plainly where the tables stop applying.

Deck load path

Deck length

Deck width (the joist span)

Live load (IRC base is 40)

psf

Dead load

psf

Joist spacing

Posts under the beam

no.

Soil bearing (optional, 0 to skip)

psf

Live load is yours to enter. The consequences of changing it are set out below.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

The load path, and the same pounds at every stage

Worked example

A 16 ft by 12 ft deck at the IRC base load, joists at 16 in on centre, three posts under the beam.

Design load. 40 live + 10 dead = 50 psf, so 192 sq ft carries 9,600 lb.
Per joist. Each joist carries 16 in of width across a 12 ft span: 800 lb.
Per beam bay. The beam takes half the joist span along its length, so each of the two bays carries 2,400 lb.
Per footing. 1,600 lb each, and that is the number a footing size actually depends on.
Now change live load to 60 and the warning fires: you are outside the load the R507 span tables were computed at.

That last step is the point of the page. Raising the live load does not just raise the numbers, it invalidates the tables you were reading spans out of.

The formula

Tributary area, four times over. Each member carries the area that drains to it:

w = live + dead joist = w · (s/12) · W bay = w · (L/bays) · (W/2) footing = w · A / 2n

w: design load, psf
s: joist spacing, in
W: joist span, ft
L: deck length, ft
bays: spaces between posts, one fewer than the posts
A, n: deck area and post count. The beam carries half the deck; the ledger carries the other half

The halving in the last two terms is the one people miss. A ledger-hung deck splits its load between the house and the beam, so the footings carry half the deck, not all of it. A freestanding deck has two beams and splits it differently.

The tables were computed at 40 psf and they do not scale

The interesting thing about deck live load is not the number. It is what happens to everything downstream when a jurisdiction changes it, and the clearest documentation of that anywhere is a Washington State Building Code Council interpretation issued at the request of the City of Seattle.

Washington amended Table R301.5 for decks and balconies from 40 psf to 60. Somebody then asked the obvious question: does that amendment carry through to the R507 joist, beam and ledger tables, which are all built on 40 psf? The answer was no, it does not, and the council had to publish replacement tables computed at 60.

The magnitudes are worth knowing even if you are nowhere near Washington. Joist spans fell by an average of 11% and beam spans by 8%, but ledger bolt spacing tightened by an average of 38%, with through-bolt spacing down 42%. The connection that fails moves furthest.

So live load is an input on this page, and the page warns rather than quietly recomputing when you push it above 40. Publishing a scaled span from an unscaled table would be exactly the kind of number this site exists not to publish.

Why this page will not print one live load number

The IRC base figure for a deck is 40 psf live plus 10 psf dead. It is not universal. Washington State amended it, and its own code council was asked what that meant for the span tables:

The state amended the live load in Table R301.5 for decks and balconies from 40 pounds per square foot (psf) to 60 psf. The 2015 IRC Section 507 - Exterior Decks, includes joist and beam span tables, and a ledger connection table, which contain values for live load on decks and balconies based on 40 psf.
Washington State Building Code Council, State Building Code Interpretation No. 16-11, requested by the City of Seattle, 2015 IRC as adopted in Washington State

The answer is the part worth reading twice:

No, the state amendment does not include amended tables. However, the revised tables included in this interpretation have been calculated assuming a 60 psf live load.
Washington SBCC Interpretation 16-11

So in that state the live load is 60 psf while the joist, beam and ledger tables printed in the code are still built on 40. The interpretation had to publish replacement tables, and it quantified what changes:

What changes	Effect at 60 psf	Spread
Joist spans	reduced an average of 11%	range 19% decrease to 19% increase, median 15% decrease
Beam spans	reduced an average of 8%	range 30% decrease to 13% increase, median 5% decrease
Ledger bolt spacing	reduced an average of 38%	range 17% to 48% decrease, median 44% decrease
Lag bolt spacing	reduced an average of 29%	range 27% to 30% decrease, median 28% decrease
Through-bolt spacing	reduced an average of 42%	range 17% to 48% decrease, median 47% decrease

Ledger bolt spacing moves most, by an average of 38%. That is the connection that fails when decks collapse, and it is the one nobody re-checks after a load amendment.

Which is why live load is an input on this page rather than a constant, and why the span tables on this site state the load they were computed at. Ask your building department what live load your jurisdiction uses before you size anything.

Frequently asked questions

How much weight can a deck hold?: A deck designed to the IRC base carries 40 psf live plus 10 psf dead, so a 16x12 deck is designed for about 9,600 lb spread evenly. That is a design load, not a test result: it says what the deck was sized for, not what it will fail at.
Is deck live load always 40 psf?: No. 40 psf is the IRC base figure in Table R301.5. Washington State amended it to 60 psf for decks and balconies, and other jurisdictions may too. Ask your building department which figure applies before sizing anything.
If my jurisdiction requires 60 psf, can I still use the IRC span tables?: No. The Washington State Building Code Council was asked exactly this and answered that the amendment does not include amended tables, so it published replacements computed at 60 psf. Joist spans fell about 11% and ledger bolt spacing tightened about 38%.
What is the difference between live load and dead load?: Dead load is the deck itself: boards, joists, beam, railing. Live load is everything you put on it, people and furniture. The IRC assumes 10 psf dead for a typical wood deck and sets the live load separately.
How much load goes on each deck footing?: On a ledger-hung deck the house takes about half and the beam takes the other half, so each footing carries roughly the deck area times the design load, divided by twice the number of posts. On a 16x12 at 50 psf with three posts that is about 1,600 lb each.
What soil bearing capacity should I use?: Whatever your building department or soils report gives you. It is a site property, not a lookup, and this page leaves the field at zero rather than guessing. Presumptive values in the code exist but vary by soil class and jurisdiction.
Does a hot tub change the calculation?: Completely, and this page does not handle it. A filled hot tub is a concentrated load far above 40 psf over a small area and needs its own framing design, usually an engineer's.

Check these numbers yourself

Washington State Building Code Interpretation No. 16-11Washington State Building Code Council, requested by the City of Seattle. The 60 psf amendment and the replacement tables it forced.
IRC R301.5 Live loadUpCodes. Table R301.5, minimum uniformly distributed live loads. Confirm the edition and jurisdiction it serves you.
AWC DCA 6: Prescriptive Residential Wood Deck Construction GuideAmerican Wood Council, free PDF. The prescriptive deck guide behind the IRC deck provisions.

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 2026-09-07. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Design loads, not capacity. These figures say what a deck is being sized for, not what an existing deck will carry. An existing deck's capacity depends on its condition, its connections and its ledger, and only an inspection can establish it. Hot tubs, planters and crowds need their own design.