Deck Load Calculator
A deck load calculator adds live and dead load, then multiplies by area: a 12 × 16 ft deck at 40 + 10 psf must carry 50 psf, or 9,600 lb in total.
How much weight does a deck need to hold?
This deck load calculator adds live load and dead load to get the design load in pounds per square foot, then multiplies by the deck area for the total load. The formula is design load (psf) = live + dead, and total load (lb) = length × width × design load. A 12 × 16 ft deck at 40 psf live plus 10 psf dead must carry 50 psf, or 9,600 lb across the whole deck.
Enter the deck length and width in feet and the live and dead loads in psf. The calculator shows the total load in pounds, the design load in psf, and the area.
How the formula works
Picture the deck divided into one-foot squares. Each square must hold its share of people and furniture, which is the live load, plus its share of the deck’s own wood and hardware, which is the dead load.
- Area = length × width.
- Design load = live load + dead load, in psf.
- Total load = area × design load, in pounds.
The psf number drives joist and beam sizing in span tables. The total load shows how much weight ends up going down through the posts and footings.
Worked examples
- Default: 12 ft × 16 ft, 40 psf live, 10 psf dead
- Area: 12 × 16 = 192 sq ft
- Design load: 40 + 10 = 50 psf
- Total load: 192 × 50 = 9,600 lb
- Snow-country deck: 10 ft × 20 ft, 60 psf live, 10 psf dead
- Area: 10 × 20 = 200 sq ft
- Design load: 60 + 10 = 70 psf
- Total load: 200 × 70 = 14,000 lb
The higher live load here stands in for a heavier local snow load. Your building department sets that number.
Where the load goes
Take example 1 attached to the house with a ledger, joists spanning 12 ft out to a beam on three posts:
- Each joist rests half on the ledger and half on the beam, so the beam carries 6 ft of deck.
- Beam load: 16 ft × 6 ft × 50 psf = 4,800 lb.
- With posts at 0, 8, and 16 ft, the middle post carries 8 × 6 × 50 = 2,400 lb and each end post 1,200 lb.
That is the load each footing must spread into the soil. The footing size calculator takes it from there.
Typical deck loads
| Situation | Live load (psf) | Dead load (psf) | Design load (psf) |
|---|---|---|---|
| Standard residential deck | 40 | 10 | 50 |
| Deck with heavier decking or railing | 40 | 15 | 55 |
| Snow region (example) | 60 | 10 | 70 |
| Stone or tile on a mortar bed | 40 | 25 or more | 65 or more |
The 40 psf figure is the IRC residential minimum. The other rows are illustrations, not code values. Check local code.
Total load by deck size at 50 psf
| Deck size | Area | Total load |
|---|---|---|
| 10 × 10 ft | 100 sq ft | 5,000 lb |
| 12 × 12 ft | 144 sq ft | 7,200 lb |
| 12 × 16 ft | 192 sq ft | 9,600 lb |
| 16 × 20 ft | 320 sq ft | 16,000 lb |
| 16 × 24 ft | 384 sq ft | 19,200 lb |
How to choose your inputs
- Length and width: the outside dimensions of the deck surface, in feet.
- Live load: start at 40 psf, then ask your building department about snow.
- Dead load: 10 psf for normal wood framing. Weigh heavy finishes separately.
- Point loads: hot tubs, big planters, and outdoor kitchens are concentrated. They need their own check.
Common mistakes
- Using the total pounds to size a joist. Joists are sized from psf and span, not from the deck total.
- Forgetting the ledger. Half of a ledger-attached deck’s load hangs on the house connection. It must be properly bolted or lagged.
- Adding a hot tub later. A deck built for 50 psf is not built for a tub.
- Ignoring railings. Guards have their own load requirements for people leaning on them.
Related calculators
Size beams with the beam load calculator and joists with the deck joist calculator. For snow, the snow load calculator converts ground snow to roof-style loads.
This is a planning estimate. Decks carry people above the ground, and failures cause serious injuries. Have a licensed structural engineer or your building department confirm framing, connections, and footings.
Frequently asked questions
What is the difference between live load and dead load?
Live load is weight that comes and goes: people, furniture, grills, planters, and snow. Dead load is the deck's own permanent weight: decking, joists, beams, railings, and hardware. This calculator adds the two, so 40 psf live plus 10 psf dead gives a 50 psf design load.
Why is 40 psf a common live load for decks?
The US International Residential Code sets 40 psf as the minimum live load for residential decks, and most deck span tables are built around it. Local amendments, heavy snow, or concentrated loads such as a hot tub can require more, so check local code for your area.
Is 10 psf enough for dead load?
10 psf covers typical wood framing with wood or composite decking. Heavier finishes add more: stone or tile set on a mortar bed, a built-in kitchen, or big masonry planters can push dead load well above 10 psf. Add up the weight of heavy items and divide by the area they sit on.
Can my deck hold a hot tub?
Usually not without extra framing. Add the tub's dry weight, the water (gallons × 8.34 lb), and the people, then divide by the tub's footprint. A tub holding 400 gallons carries 3,336 lb of water alone; on a 7 × 7 ft footprint of 49 sq ft, that is 68 psf before the shell or occupants. Most tubs need a dedicated, engineered support or a slab.
Does a bigger deck mean a bigger load per square foot?
No. The psf figure stays the same; it describes how hard each square foot is used. A bigger deck has a bigger total in pounds only because it has more square feet. A 12 × 16 deck at 50 psf carries 9,600 lb; a 16 × 24 deck at the same 50 psf carries 19,200 lb.
Estimates only. Verify quantities with your supplier before purchasing.