Deck Load Calculator

This calculator determines the size of each tributary area of your deck. It then determines the total load from each tributary area based on the design load of your deck. Default is 50 psf which you can change.

The total load for each tributary area is then divided by the area of each footing in order to determine the load psf imposed on the soil.

This value must not be greater than the bearing capacity of the soil. If it is, you must increase the size of the footing to spread the weight out over a greater surface area or increase the number of footings.

For help, simply click on the(?) beside the section you need help with, Or watch this Tutorial Video.

Select between either a deck that attaches to the ledger of the house or a free standing deck supported by beams only.

This calculator is designed to work only for square or rectangular shaped decks. More complex shapes are beyond the scope of this calculator.

Single Ledger
Freestanding Deck

Select the approximate soil bearing capacity based on pounds/square foot (psf).

  • Crystalline bedrock - 12,000 psf
  • Sedimentary rock - 4,000 psf
  • Sandy gravel or gravel - 3,000 psf
  • Sand, silty sand, clayey sand, silty gravel, and clayey gravel - 2,000 psf
  • Clay, sandy clay, silty clay and clayey silt - 1,500 psf

Don't know what soil you have? Try these tests.

It can be challenging determining what kind of soil you have if there is clay in it. These tests will give you an general idea.

Dirt ball test

Squeeze some soil together about the size of a snow ball and drop from a height of 1 foot. If you can't make a ball or if it crumbles on impact it has a low percentage of fine clay. If it holds together well on impact, it has a high percentage of cohesive clay.

Water test

Place a ball of soil into a large glass jar of water and observe the sediments. Gravel and sand will settle quickly. Finer sediments will take 20 minutest to an hour to settle. Clay will disolve and may take 24 hours or more to settle and the water will remain cloudy.

Noodle test

Try to roll a piece of soil into a 1" diameter noodle like a piece of spaghetti. If you can do this without it breaking apart you can assume it has a high percentage of clay.

Soil Bearing Capacity:

Select the number of beams you intend to use to support your deck structure. Beams always run perpendicular to the direction of the joists.

Also select the number of support posts that the beam will rest on. The support posts then sit on piers or footings which distribute the load to the soil.

Number of Supporting Beams: (not including ledger)
Number of Support Posts per Beam:

The design load is the load value that the deck structure is intended to support. It is comprised of the live load and dead load. Live load accounts for people and things on a deck. Dead load accounts for the weight of the structure itself.

The default value is 50 psf (lowest value allowed) and comprises a dead load of 40 psf and a live load of 10 psf. You may adjust this higher as you wish.

Design Load (suggested 50lbs/sqft):

Select the shape of the footing that distributes the load from the deck structure to the soil.

Sometimes cylindrical concrete piers have rectangular footings beneath them with spread out the load. Make your selection based on the shape of the foundation that distributes the final load to the soil.

Footing Type: Round | Rectangular