Decking Area Calculator

Calculate decking area for real-world deck shapes. Add up to 12 rectangular sections, because L-shaped and multi-level decks are just rectangles added together: the tool returns the total area in square feet, square yards and square metres, the material cost at your price per square foot, and, with a waste factor, the area to actually buy and its cost.

Quick Answer

How to use this calculator

The direct answer: split the deck into rectangles, enter length and width for each section in feet, enter your material price per square foot and a waste factor, then press Calculate. Start with the two pre-filled sections, press Add section for each extra rectangle up to 12, and use Remove on a section you no longer need. One quirk to know before you remove anything: Remove copies the last section into the slot you removed, so check the remaining sections after a removal instead of assuming the list simply shrank.

Blank or zero sections are skipped silently, so a half-filled row will not break the total, but it will not warn you either. A negative dimension, a negative price, or a negative waste factor stops the calculation with an error, and a blank waste field is treated as 0%, not as the 10% default. Results stay hidden until you calculate, and Reset restores the worked example. Once you have the area, our deck cost calculator turns it into a full project budget, which is the next step in the pipeline, not a rival answer.

Worked example, checked against this exact tool

Two sections: 20 by 12 ft and 10 by 8 ft, material at $12 per sqft, waste 10%. Enter both sections: total = 240 + 80 = 320.00 sqft (35.56 sq yd, 29.73 sq m); material cost = $3,840.00; to buy = 352.00 sqft = $4,224.00. These exact figures are what the calculator above produces, and the formula panel under the result shows the same arithmetic so you can check it by hand.

Why rectangles beat freehand shapes

The direct answer: almost every deck decomposes into rectangles, so a multi-section tool matches how decks are actually built. An L-shape is two rectangles, a multi-level deck is one per level, and a deck around a pool is the bounding box minus the pool cutout, entered as separate rectangles. Measure each rectangle's longest length and width, add a section per piece, and the tool does the addition.

For genuinely curved edges, measure the bounding rectangle and accept the slight over-estimate: it becomes part of your waste factor. That over-estimate is honest, because a curved edge still has to be cut from rectangular boards, and the offcuts are waste by another name. To go from area to a shopping list, our deck material calculator converts square footage into boards.

The waste factor is not optional

The direct answer: decking boards come in fixed lengths, so every cut, angle, and board end wastes material, and pricing the net area under-buys and stalls the project a week from done. Ten percent is the standard planning figure for rectangular layouts, 15% for diagonal patterns or complex shapes, and the buy line prices what you actually order.

Competitor guides agree on the shape of that rule while differing on the exact band: simple rectangles need the least, L-shapes sit in the middle, and diagonal or complex layouts need the most. That spread is the reason the tool lets you set the factor instead of hard-coding it. If your layout has a picture-frame border, a herringbone section, or many short runs, step the factor up before you order, because the cost of one extra bundle is small against the cost of a stalled job.

Common deck sizes, costed

The direct answer: at the worked-example price of $12 per sqft with 10% waste, a 12 by 12 ft deck costs $1,900.80 to buy, a 16 by 16 ft deck costs $3,379.20, and a 20 by 12 ft deck costs $3,168.00. Those are material budgets only, computed by the same tool, and your supplier's price replaces the $12 example figure in every row.

These are material budgets only: installation labor, framing, and railing are separate. Run your own dimensions above for the exact buy quantity. Maintaining rather than rebuilding? Our deck refinishing cost calculator prices the refinish route.

What the area does not tell you

The direct answer: area tells you the decking surface, and almost nothing else about the project. Board counts depend on board width, gap, and layout direction; framing lumber, fasteners, and railing are separate take-offs. Use this area for material budgeting and quotes, then let the supplier convert it to lineal feet and board counts for the order.

Stairs and railing deserve the same separation. Stairs need treads, risers, and stringers beyond the surface area, so measure them separately rather than folding them into a rectangle. Railing runs the deck perimeter, and a good rule is to measure the outer edge of every exposed side and price it per lineal foot from your supplier, which our linear foot calculator can help price. None of these belong in the area total: keep them as separate line items in the budget.

What competing calculators miss

The direct answer: most rival tools handle one rectangle, or bury area inside a project estimator, so the multi-section deck that people actually build has to be worked out by hand. Single-rectangle tools force an L-shaped deck into a bounding box, which overcharges for space that is not deck, and project estimators hide the area line inside labor, railing, steps, and permits, so you cannot check the shopping list.

The gaps that matter most are waste and board conversion. Several multi-rectangle tools price the net area with no waste factor, which under-buys by exactly the amount the job will need, and several board tools ask for board width and length but never show the waste-adjusted buy quantity next to the net area. This page keeps the multi-section hero, prices what you actually buy, and shows the board conversion below as prose, so the area, the buy quantity, and the board count all reconcile.

From square feet to a shopping list: board math

The direct answer: lineal feet equal area in sqft times 12 divided by board width plus gap, both in inches. Each board covers its face width plus one gap, so the conversion is lineal feet = area x 12 / (board width + gap, in inches). For standard 5.5-inch decking with a 1/4-inch gap, each board covers 5.75 inches: 320 sqft x 12 / 5.75 = 667.8, about 668 lineal feet.

With 12-foot boards that is 668 / 12 = 55.7, so 56 boards before waste; apply the 10% waste factor to the lineal feet and round up, and order 62. Gap width matters more than it looks: a 1/8-inch gap instead of a 1/4-inch gap changes the board count by about 2%, enough to matter on a big deck. For 300 square feet the same arithmetic gives about 626 lineal feet, 53 twelve-foot boards before waste, and 58 boards with 10% waste.

Cutouts, angles, and diagonal layouts

The direct answer: measure cutouts as rectangles the deck works around, not through. For a pool or bay window inside the deck footprint, split the deck into the rectangles that surround the obstacle and enter each as a section; never enter the full bounding box and hope the numbers sort themselves out.

For angled or clipped corners, measure the bounding rectangle and let the waste factor absorb the over-estimate, since the offcuts from the angle become waste anyway. Diagonal and herringbone layouts change the waste math, not the area math: boards cut at 45 degrees produce far more offcuts than boards run square, so plan 15% waste for diagonal patterns and more for complex shapes, and price the buy quantity accordingly.

Sloped yards and raised decks: measure the surface, not the ground

The direct answer: area is the walking surface, so on a sloped yard measure the deck boards, not the dirt underneath. A tape run along the slope reads longer than the horizontal deck it supports; measure level, or measure the finished deck itself. Raised decks follow the same rule: the area is the platform you stand on, and the posts, footings, and skirting are separate line items, exactly like stairs and railing.

Permits usually ask for the plan area too, which is the same number from above. If the approved plan shows 320 sqft and your sections add to 384, something was measured twice or a cutout was forgotten; reconcile before ordering, because the buy quantity and the permit should agree.

Frequently asked questions

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