How to Calculate Deck Stairs: A UK Guide

Every set of stairs comes down to four figures. The total rise is the vertical height from the ground (or lower deck) to the finished surface of the top deck. The riser height is the vertical height of each individual step. The going is the horizontal depth of each tread that your foot actually lands on. And the stringer length is the diagonal length of the timber that carries the steps, which is what you need to know before you buy and cut a board.

Everything starts from the total rise, because it's the one figure the site gives you and you can't change. The rest you solve for.

Takeaway: measure the total rise accurately to the finished deck surface first, because every other number depends on it.

In England and Wales, stairs are covered by Building Regulations Approved Document K. For a private stair serving a dwelling, the numbers that matter are a maximum riser height of 220 mm, a minimum going of 220 mm, and a rule that ties them together: twice the rise plus the going (2R + G) should fall between 550 mm and 700 mm. There's also a maximum pitch of 42 degrees for a private stair.

External and garden steps are treated a little more leniently in practice, and many deck builders work to a shallower, more comfortable rise around 150 to 180 mm because outdoor steps are often taken at pace and in poor light. But the Approved Document K figures are the sensible envelope to stay inside, and if your deck stairs form the main access to the dwelling, they apply properly.

Takeaway: keep the riser at or below 220 mm, the going at or above 220 mm, and 2R + G between 550 and 700 mm, and you're inside the UK envelope.

You can't have a fraction of a step, so the first job is to divide the total rise into a whole number of equal risers. Take your total rise and divide it by a target riser height, then round to the nearest whole number.

Say the target is 180 mm, a comfortable outdoor rise. Divide the total rise by 180, round it, and that's your riser count. Then — and this is the step people skip — recalculate the actual riser height by dividing the total rise by that whole number of risers. It won't be exactly 180, and it shouldn't be: every riser must be identical, because uneven risers are the single most common cause of trips on stairs. The calculator enforces this automatically, but by hand it's the part to be careful with.

Takeaway: round to a whole number of risers first, then divide back to get the exact equal riser height — never leave a taller or shorter last step.

The number of treads is always one fewer than the number of risers, because the top deck surface itself is the last "step." Multiply the number of treads by your going to get the total run, which is how far the stairs project horizontally out from the deck. This figure matters more than people expect on a small garden, because a tall deck can throw the bottom step a surprising distance into the lawn.

Takeaway: treads equal risers minus one, and total run equals treads times going — check that run fits your garden before you commit.

The stringer is the diagonal, so it's simple Pythagoras. Square the total rise, square the total run, add them, and take the square root:

Stringer length = √(total rise² + total run²)

Add a bit at each end for the connection at the top and the footing at the bottom, and buy the next stock length up. The stair angle, if you want it, is the arctangent of rise divided by run, and for a UK private stair it needs to come out at 42 degrees or less.

Takeaway: the stringer is the hypotenuse of the rise and run, so work it out with Pythagoras and buy the next stock length up to allow for cuts.

A raised deck stands 1,080 mm above the lawn. We'll set out the stairs to a comfortable outdoor rise.

Two things separate a deck stair that lasts from one that doesn't, and neither is in the arithmetic.

First, ignore the American 7-11 rule. It produces a rise of 178 mm and a going of 279 mm, which isn't dangerous, but it's optimised for interior US stairs and it makes UK deck stairs longer in the run than they need to be, eating garden for no comfort gain. Set out to UK figures: a rise around 170 to 180 mm and a going around 250 mm is a genuinely comfortable outdoor stair that also stays inside Approved Document K.

Second, the number of stringers is a structural decision, not a material-estimating one. It's tempting to treat "number of stringers" as just a quantity for the timber order, but on a flight wider than about 900 mm, two outer stringers will let the treads flex and bounce underfoot, and over a few winters of wet-dry cycling that movement loosens fixings and splits boards. Add a central stringer on anything wider than 900 mm, and space stringers no more than about 400 to 450 mm apart. A springy step feels cheap and it fails early.

Takeaway: set out to UK rise and going rather than the US 7-11 rule, and add a centre stringer on any flight over 900 mm wide.

Once the layout is right, the order of work is: cut and check one stringer, use it as a template to mark the rest so every notch is identical, then fix the stringers at the top to the deck frame and at the bottom to a solid footing or paving so they can't kick out. Use treated C24 timber for anything outdoors, and re-treat every freshly cut end with end-grain preserver, because the saw exposes untreated wood that's the first place rot starts.

Takeaway: cut one stringer, template the rest from it, and re-treat every cut end before it goes outside.