For a rectangular deck, square footage is length times width: a 16 × 12 ft deck is 192 sq ft. For any other shape, split the outline into rectangles, triangles and circle segments, find the area of each, and add or subtract them. The math is simple; the mistakes usually come from measuring the wrong edge or mixing feet and inches.
Below is how I measure an existing or planned deck, with a worked example for each common shape.
What you are actually measuring
Before you pull a tape, decide which area you need, because a deck has more than one.
- Decking surface. The area of the walking surface, measured to the outer edge of the boards. Use this for deck boards, stain and sealer.
- Framing footprint. The area inside the outer rim joists. It is slightly smaller than the surface, since boards usually overhang the rim by an inch or so. Use this for joists, beams and footing layout.
- Stairs and landings. Measured separately. They add board and stain area but are not usually counted as deck floor area for a permit.
For a quick estimate the difference between surface and footprint is small. For a finish estimate, it matters on boards with a picture-frame border, where the border boards overhang and are counted separately.
Measuring tools and technique
You need a 25-ft or longer tape, a pencil, a notepad with a rough sketch, and ideally a second person to hold the dumb end of the tape.
A few habits that prevent errors:
- Sketch first, then measure. Draw the outline, label each edge with a letter, then write the measurement next to each letter.
- Measure every edge, even ones that “should” match. Old decks are rarely perfectly square.
- Write feet and inches, then convert. Do not round 11’9” to 12 ft on the notepad. Convert at the end.
- Measure the diagonals. On a rectangle, the two diagonals should be equal. If they differ by more than about 1/2”, the deck is out of square and your area may be slightly off.
Converting inches to decimal feet
Divide inches by 12. These come up constantly:
| Inches | Decimal feet |
|---|---|
| 3” | 0.25 |
| 4” | 0.333 |
| 6” | 0.5 |
| 8” | 0.667 |
| 9” | 0.75 |
| 10” | 0.833 |
So 16’4” is 16.333 ft, and 11’9” is 11.75 ft.
Rectangles and squares
Area = length × width.
Worked example. A planned deck measures 16 ft along the house and 12 ft out.
- 16 × 12 = 192 sq ft
With inches. An existing deck measures 16’4” × 11’9”.
- 16.333 × 11.75 = 191.9 sq ft
Round up to the next whole square foot for ordering, so call it 192.
L-shaped and multi-section decks
Split the shape into rectangles that do not overlap, find each area, and add them.
Worked example. A main deck of 20 × 12 ft with an 8 × 6 ft bump-out for a grill.
- Main section: 20 × 12 = 240 sq ft
- Bump-out: 8 × 6 = 48 sq ft
- Total: 240 + 48 = 288 sq ft
There is usually more than one way to split an L. Any split gives the same total as long as the pieces do not overlap or leave gaps. If you get different totals from two splits, one of the edge measurements is wrong.
Clipped (angled) corners
Many decks have 45° corners cut off the outer edges. Measure the full rectangle as if the corners were square, then subtract a triangle for each clip.
For a 45° clip, the two legs of the removed triangle are equal. Measure along the original edges from where the angle starts to where the corner would have been.
Triangle area = leg × leg ÷ 2.
Worked example. A 16 × 12 ft deck with both outer corners clipped 3 ft back on each side.
- Full rectangle: 16 × 12 = 192 sq ft
- Each triangle: 3 × 3 ÷ 2 = 4.5 sq ft
- Two triangles: 9 sq ft
- Total: 192 − 9 = 183 sq ft
If you can only measure the angled edge itself, a 45° clip with legs of L has an angled edge of about 1.414 × L. A 4’3” angled edge (4.25 ft) means legs of about 3 ft.
Octagons, circles and curves
Regular octagons
For an octagon with eight equal sides of length s:
Area ≈ 4.828 × s²
Worked example. An octagonal deck with 6-ft sides.
- 6 × 6 = 36
- 36 × 4.828 = 173.8 sq ft
If the sides are not equal, treat it as a rectangle with four clipped corners and use the method above.
Circles and half-circles
Circle area = 3.1416 × radius². A half-circle is half of that.
Worked example. A 16 × 12 ft deck with a half-circle bump-out 8 ft across (radius 4 ft) on the outer edge.
- Rectangle: 192 sq ft
- Half-circle: 3.1416 × 4 × 4 ÷ 2 = 25.1 sq ft
- Total: 217.1 sq ft
Irregular curves
For a free-form curve, draw a straight baseline along the flat side and measure out to the curve every 12” or 24”. Treat each strip as a rectangle using the average of its two edges, and add the strips. More strips give a better estimate. For anything this complex, expect to add extra material for cutting, since curved edges create a lot of offcuts.
Multi-level decks and stairs
Measure each level as its own shape and add them. Do not forget the step-down boards between levels; they are usually a single board run along the full length of the transition.
For stairs, count the treads and multiply by width and tread depth.
Worked example. Stairs 4 ft wide with 3 treads, each 11” deep.
- 11 ÷ 12 = 0.917 ft
- 4 × 0.917 × 3 = 11 sq ft of tread surface
Add riser boards if you are covering them. The deck stair calculator works out tread and riser counts for a given deck height.
Turning square footage into materials
Square footage is the starting point for most material estimates, but it is not the whole story.
Deck boards
Area alone gives a rough board count. TimberTech’s installation guide, for example, says to divide deck square footage by 5.5 for 12-ft boards, 7.3 for 16-ft boards and 9.1 for 20-ft boards, then add 10% for scrap (TimberTech installation guide). For 192 sq ft in 16-ft boards: 192 ÷ 7.3 = 26.3, round to 27, plus 10% = 30 boards.
That quick method does not know whether your deck dimension matches the board length. A 17-ft run uses very different material from a 16-ft run with the same area. A row-by-row count, like the one our deck board calculator uses, accounts for board length, actual board width and gap.
Waste also depends on layout. Diagonal and picture-frame patterns produce more offcuts than straight runs; a common planning allowance for 45° diagonal layouts is about 15%, though manufacturers do not publish pattern-specific waste figures.
Stain and sealer
Stain coverage is quoted per gallon per coat. Typical first-coat coverage runs about 150 to 300 sq ft per gallon depending on the product and how thirsty the wood is (Angi deck stain calculator). Include stair treads, risers and railings in the area, since railings can add a surprising amount of surface. The deck stain calculator adds coats and railing area for you.
Cost
Contractors and cost guides often quote per square foot of deck. That works for comparing bids, but check whether a quoted rate includes railings, stairs and demolition. The deck cost breakdown separates those lines.
Common measuring mistakes
- Using the house wall length instead of the deck length. The deck may stop short of a corner or extend past it.
- Mixing feet and inches. 12’6” is 12.5 ft, not 12.6 ft.
- Measuring inside the rim for board estimates. Boards cover the full surface including overhang.
- Forgetting cutouts. Subtract hot tub openings, tree cutouts and built-in planters if they are not decked over.
- Double-counting overlap where two rectangles in an L share an area.
For quick results on any of these shapes, the deck square footage calculator handles rectangles, L-shapes and clipped corners.
Frequently asked questions
Do I include the stairs in deck square footage?
For a permit application or a size comparison, usually not; most people quote the main deck floor area. For materials such as boards and stain, include the treads and risers as a separate line. Your building department can tell you how it counts area for permit purposes.
Should I measure to the edge of the boards or the frame?
Measure to the board edge for decking, stain and sealer, since that is the surface you are covering. Measure the frame, inside the rim joists, for joist and footing layout. The difference is small on most decks but adds up on large or picture-framed ones.
How do I measure a deck that is not square?
Measure every edge and both diagonals, then split the shape into triangles if needed. For a four-sided shape that is slightly out of square, averaging opposite sides and multiplying gives a close estimate. For ordering materials, rounding up a few square feet is cheaper than a second trip to the lumberyard.
How much extra should I add to square footage when ordering boards?
TimberTech’s own method adds 10% for a standard straight layout. Diagonal, herringbone and picture-frame layouts generate more offcuts, so many estimators add 15% or more for those. A board-by-board count based on your actual dimensions is more reliable than a flat percentage.