Deck Ramp Calculator

Worked example

With these inputs:

• Total rise (deck surface to ground): 24 in
• Slope: 1:12 (8.3%) — code maximum for wheelchairs
• Ramp width (outside of framing): 48 in
• Rules to check: IRC — ramp at a house (R311.8)

IRC R311.8.1: 1:12 max for the ramp serving the required exit door, 1:8 for other ramps; R311.8.2: 3 ft × 3 ft landings at the top, the bottom, at doors and at turns. ADA 405: 1:12 max, 30 in max rise per run, 60 in landings, 36 in clear width, handrails both sides above 6 in of rise. Your local code and any accessibility rules for your use still apply.

How it works

A ramp is a long, shallow stair: every inch of rise needs a set length of run, so the height of the deck decides how long the ramp is. This works out the run for the slope you choose, the landings the code asks for, the sloped length you frame and deck, how many stringers the width takes and whether handrails are required — under the house code (IRC R311.8) or the stricter ADA Standards for an accessible route.

Frequently asked questions

How long does a ramp need to be?

Multiply the rise by the slope ratio. At the 1:12 wheelchair maximum, every inch of rise needs 12 inches of run, so a deck 24 in above the ground needs a ramp 24 ft long, plus landings. At 1:20 the same deck needs 40 ft. A 1:8 ramp is only 16 ft but is too steep for most wheelchair users.

What is the code for a ramp at a house?

The IRC (R311.8) limits the ramp that serves the required exit door to 1:12 and other ramps to 1:8, requires a 3 ft by 3 ft landing at the top and bottom, where a door opens onto the ramp and where it turns, and requires a handrail on at least one side when the ramp is steeper than 1:12. Local amendments can be stricter.

How many feet of ramp do I need for 3 steps?

Three steps at about 7 in each is roughly 21 in of rise, so a 1:12 ramp needs about 21 ft of run. Under the ADA rules that is still one run, because each run may climb up to 30 in before it needs a 60 in landing.

When does a ramp need handrails?

Under the ADA Standards, any ramp run that rises more than 6 in needs handrails on both sides, 34 to 38 in high. Under the IRC, a house ramp needs a handrail on at least one side once it is steeper than 1:12. Any open side more than 30 in above the ground also needs a guard.

How wide should a deck ramp be?

The ADA Standards require at least 36 in clear between the handrails, so frame the ramp about 42 to 48 in wide to leave room for posts and rails. The IRC sets no ramp width for houses, but 36 in clear is the practical minimum for a wheelchair.