Calculator
How to use
- Select your deck outline: rectangle, circle, or custom sides.
- Choose units and enter dimensions measured along the outer edge.
- Optional: subtract gate or stair openings to estimate edging length.
- Set a waste factor for cuts, corners, and small mistakes.
- Add post spacing and standard board length for quick counts.
- Press calculate to view results above this form.
- Download CSV for spreadsheets or PDF for onsite planning.
Example data table
| Scenario | Inputs | Output |
|---|---|---|
| Rectangle deck | Length 4 m, Width 3 m, Waste 7% | Raw 14.00 m, Order ≈ 14.98 m |
| Round deck edge | Diameter 3 m, Waste 10% | Raw ≈ 9.42 m, Order ≈ 10.36 m |
| Custom outline | Sides 2,2,2,1.5,1.5 m, Waste 8% | Raw 9.00 m, Order ≈ 9.72 m |
| With opening | Rectangle 4×3 m, Gate 0.9 m, Waste 7% | Effective 13.10 m, Order ≈ 14.02 m |
Formula used
The calculator first finds the perimeter of the chosen outline. For a rectangle, it uses P = 2(L + W). For a circle, it uses P = πd or P = 2πr. For a custom outline, it sums all side lengths.
If you subtract openings (gate or stairs), it applies Peffective = max(0, P − openings). Then it adds waste for ordering: Porder = Peffective × (1 + waste%/100).
Planning helpers convert that ordered perimeter into counts: posts ≈ ceil(Porder/spacing)+1, boards ≈ ceil(Porder/board length). Border strip area is approximated as Peffective × border width.
Perimeter planning for garden-facing decks
Accurate perimeter estimates guide edging, trim, and garden borders that follow the deck line. Measure the outside edge, not joist spacing. For rectangles, confirm corners are square and recheck both dimensions. For curves, confirm whether you measured radius or diameter. For irregular outlines, record each straight segment. The calculator converts those inputs into one perimeter you can order from confidently.
Allowances for cuts, joints, and site conditions
Material takeoffs need allowances for cuts, miters, joints, and minor framing errors. Corner returns and splice joints create offcuts that may not be reusable. Add a waste percentage when ordering boards, fascia, or edging so you avoid delays. Use higher waste for complex angles, composite trims, or frequent splices. The tool applies waste after openings are removed, matching typical estimating practice.
Openings for stairs and access points
Stairs and access points interrupt continuous runs of edging or low fencing. Subtracting these widths improves accuracy for materials that should not span the gap. Measure the opening width along the same perimeter line you measured the deck edge. Enter gate and stair openings separately when both exist. The calculator subtracts openings first, then adds waste to the remaining perimeter for ordering.
Post spacing and standard length takeoffs
Perimeter also supports quick counts for posts and standard stock lengths. If you set a post spacing, the calculator estimates the number of posts around the edge and adds one to close the loop. For trims or rails, the standard board length converts ordered perimeter into a board count using a ceiling rule. Treat these as planning numbers, then refine with a layout drawing. For guard systems, confirm code-required spacing and corner reinforcement; adjust counts for double posts at stair transitions and hardware brackets as needed onsite.
Using border strip area for planting design
When decks border beds, perimeter can translate into a planting strip for mulch, gravel, or groundcovers. Enter a border width to estimate strip area using perimeter multiplied by width. This approximation works best for narrow, consistent bands. For wide or irregular planting zones, compute area separately. Still, the strip estimate helps order weed barrier, landscape fabric pins, and bulk material.
FAQs
Should I use raw or effective perimeter for ordering?
Use effective perimeter for continuous items that stop at openings. Use perimeter with waste when buying boards, edging, or lighting runs that require cuts and extra pieces.
How do I measure a curved deck edge accurately?
Use a flexible tape along the curve, or measure radius/diameter from a known center point. If the curve is part of a circle, radius gives the best result.
What waste percentage is reasonable?
For simple rectangles, 5–8% is common. For many corners, splices, or composite trims, 10–15% is safer. Increase waste if you expect on-site adjustments.
Does post spacing here replace structural design?
No. This is a planning estimate for perimeter posts or small borders. Confirm structural requirements, guard rules, and footing layout with local codes and your deck plan.
Why does the calculator add one extra post?
A closed perimeter needs a post at the start and the end point, which coincide. Adding one helps avoid undercounting when spacing is converted to whole posts.
Can I use the border strip area for mulch or gravel?
Yes, for narrow, consistent bands around the deck. Multiply the strip area by your material depth to estimate volume, then convert using your supplier’s coverage rates.