Deck, stair and post calculators

Which outdoor calculator do I need?

Work from the ground up. A deck or a set of steps starts with footings or posts set in concrete, then framing, then the walking surface. Each layer has its own tool. The concrete per fence post calculator counts bags of post mix for each hole. The stair rise and run tool divides a total rise into equal steps that meet code. The decking boards and screws calculator counts the boards across the deck and the fasteners that hold them.

Framing itself, meaning joists, beams and ledger, is sized from published span tables rather than calculated. The deck framing and board takeoff guide shows how to read those tables and count the joists.

Deck, stair and post pages

What stair dimensions does code allow?

A stair must have equal steps within a narrow tolerance, and the limits differ between the US and the UK. IRC: largest and smallest riser (and tread) in a flight may differ by no more than 3/8 in. That tolerance is why the stair calculator divides the total rise into equal risers instead of using a fixed step height: an odd last step is a trip hazard and falls outside the tolerance.

A common belief is that a short or tall final step can be fixed by eye when the stair is built. It cannot: the variation limit applies to every riser in the flight, including the one at the bottom where the ground may not be level. Measure the total rise from the landing point, not from the deck edge to an assumed ground line. Deck stairs also have structural rules for their stringers. DCA 6: deck stair stringers must be at least 2x12. DCA 6: cut stringers at 18 in o.c. max; at least 3 for a 36 in stair. DCA 6: a cut stringer may span at most 6 ft 0 in horizontally. UK stairs are checked with a comfort formula as well as limits. Approved Document K: twice the rise plus the going must be 550 to 700 mm. A private stair may not be steeper than 42 degrees.

How are deck boards and fasteners counted?

Across the deck, by board width plus gap, and then along it, by board length. Actual widths are smaller than the names suggest. A 5/4x6 wood deck board is about 1 in thick and 5-1/2 in wide (dry, dressed). Trex Signature, Transcend and Enhance boards measure 0.94 in x 5.5 in (24 x 140 mm). TimberTech Composite Prime/Prime+ boards are 0.94 in thick and 5.36 in wide. Gaps add up across a wide deck, so the calculator includes them. Trex boards need a 3/16 in (5 mm) side-to-side gap at any temperature. DCA 6 says to space wood deck boards about 1/8 in apart.

Board layout changes framing. Diagonal (45 degree) composite decking needs joists at 12 in on center maximum. Each length of decking must bear on at least three joists. Allowances differ by maker and pattern: Trex suggests ordering 5% extra decking. TimberTech's install guide suggests about 10% extra for scrap. TimberTech suggests 30% extra for diagonal or elaborate patterns. Fastener packs are sold by area. CAMO sells EDGE screws as 350 per 100 sq ft (9.29 m2) of decking. A Trex Hideaway Universal Fastener box covers 50 sq ft; the bucket covers 500 sq ft.

How deep and wide should post holes be?

Deep enough to reach firm soil below frost, and about three post-widths across. Dig the hole about three times the post width (4 in post -> 12 in hole). Put about 6 in of gravel in the bottom of each hole for drainage. For decks the rule is set by DCA 6. Footings must bear on undisturbed soil at least 12 in below grade or below the frost line, whichever is deeper. Deck footings closer than 5 ft to the house foundation must bear at the same elevation as the house footing. Fence posts follow a proportion of their height. Concrete makers say to dig about one-third (up to one-half) of the post's above-ground height. In the UK, UK guidance: set posts at least 600 mm deep; a 1.8 m fence needs a 2.4 m post.

The post hole calculator gives the concrete for the hole size you choose. It does not size a deck footing for load; DCA 6 sizes those from beam span and soil bearing, and your building department may set its own minimum. For fence lines and garden walls, the landscaping calculators cover the rest of the job, and slabs and footings sit with all concrete tools.

How many decks are built, and how many people are hurt on them?

Patios now far outnumber decks on new US homes. About 625,000 of 1,005,000 new US single-family homes completed in 2025 (roughly 62%) included a patio, versus 180,000 with a deck and 84,000 with no patio, porch or deck (Census Bureau). About 180,000 of the 1,005,000 new US single-family homes completed in 2025 (roughly 18%) were built with a deck, alone or with a patio or porch, per the Census Bureau. On existing homes the work is steady: About 1.93 million US homeowners added a porch, deck, patio or terrace in 2023, spending about $16.0 billion in total (Harvard JCHS Table A-1, 2023 AHS). Lumber prices, which drive deck costs, have swung widely. US producer prices for softwood lumber rose 14.2% in the year to September 2026 but were still 50.4% below the May 2021 peak and only 3.0% above the 2020 average (BLS PPI).

Stairs are among the most common sources of injury in any home. An estimated 1,310,287 people (95% CI 1,082,129–1,538,444) were treated in US hospital emergency departments in 2025 for injuries involving stairs or steps (product code 1842), per our analysis of CPSC NEISS data. People aged 65 and over made up 32.3% of stair and step injuries treated in US emergency departments in 2025 (423,006 of the national estimate, 95% CI 346,762–499,249), per our analysis of CPSC NEISS 2025 data. Decks and fences appear in the same data. An estimated 151,403 people (95% CI 121,876–180,930) were treated in US hospital emergency departments in 2025 for injuries involving porches, balconies, open-side floors or floor openings (code 1817), per our analysis of CPSC NEISS data. An estimated 115,803 people (95% CI 98,955–132,650) were treated in US hospital emergency departments in 2025 for injuries involving fences or fence posts (code 1871), per our analysis of CPSC NEISS data. These estimates count emergency visits where the product was involved, not only building accidents, and they come from a sample of hospitals. CPSC's 2024 NEISS estimates come from a redesigned sample of 100 hospitals, of which 77 reported, while 2023 estimates used the older 96-hospital sample, so 2023-to-2024 changes are not clean trends (the all-product estimate jumps from 12.74 million to 15.07 million).

The how many people are hurt falling off ladders page has the full tables and confidence intervals. For prices and debris, see construction material price changes and C&D debris figures by material.

Where do these calculators stop?

They count boards, bags and steps. They do not design structure, check soil or approve a permit. DCA 6 itself has limits: DCA 6 span tables assume a 40 psf live load and a 10 psf dead load. DCA 6 limits joist span to 18 ft 0 in for its footing design. Decks outside those limits, such as roof decks, very tall decks or decks with heavy planters, need an engineer.

Stair results assume a straight flight on level ground. Winders, spiral stairs and steps that land on a slope need extra checks, and the UK has separate limits for utility and general access stairs. Read our calculation methodology for how code values are sourced and when a single source is named. A printable stair rules card for US and UK and reusable stair and deck diagrams are free to download.

Deck, stair and post questions