CalcLibrary

Decking Calculator

Estimate how many decking boards to buy for a rectangular deck surface. Enter your own deck size, board face width, gap, stock board length, and waste rate to turn rough measurements into a practical board count before shopping.

Units
m
m
mm
mm
m
%
Examples

A 4 m × 3 m deck with 140 mm boards, 6 mm gaps, 4 m stock length, and 10% waste.

Boards to buy
24
Rows needed
21
Boards per row
1
Total decking length
84 m
Deck area
12 m²
Waste-adjusted area
13.2 m²

Built from 21 rows × 1 stock boards per row, then rounded up again with 10% waste.

Surface-board estimator only. It does not cover framing, joists, footings, fasteners, structural design, seam optimization, or code compliance.

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Examples

How It Works

Formula

r=⌈WdWb+g⌉r = \left\lceil \frac{W_d}{W_b + g} \right\rceil

br=⌈LdLb⌉b_r = \left\lceil \frac{L_d}{L_b} \right\rceil

N=⌈r⋅br⋅(1+w100)⌉N = \left\lceil r \cdot b_r \cdot \left(1 + \frac{w}{100}\right) \right\rceil

Lt=r⋅LdL_t = r \cdot L_d

A=Ld⋅WdA = L_d \cdot W_d

Aw=A⋅(1+w100)A_w = A \cdot \left(1 + \frac{w}{100}\right)

Variables

LdL_d

Deck length(m)

WdW_d

Deck width(m)

WbW_b

Board face width(m)

gg

Gap between boards(m)

LbL_b

Stock board length(m)

ww

Waste allowance(%)

rr

Rows needed

brb_r

Boards per row

NN

Boards to buy after waste

LtL_t

Total decking length(m)

AA

Deck area(m²)

AwA_w

Waste-adjusted area(m²)

The calculator treats the deck as a rectangle with boards running along the entered deck length and rows stacking across the entered deck width. It divides deck width by board face width plus gap to get rows, divides deck length by the stock board length to get boards per row, multiplies those counts, then rounds up again after adding your waste allowance. Total decking length reports the net lineal coverage of the surface boards before waste; deck area and waste-adjusted area give a second check on scale.

Frequently Asked Questions

01Why does the calculator ask for board face width?
Because the math follows visible deck coverage. Enter the exposed face width you want to cover the surface with, not a nominal product label or an assumed regional standard.
02Why are rows and boards per row rounded up?
A partial course still needs a full row, and a partial run still needs a full stock board. The final board total is rounded up again after waste because the buying estimate has to end on whole boards.
03What waste rate should I enter?
Enter the rate you want to plan around. Simple rectangles, diagonal layouts, picture-frame borders, defects, and heavy trimming can all change real waste, so this tool leaves that choice to you.
04Does this include joists, framing, fasteners, or code checks?
No. This is a surface-board estimator only. It does not size framing, joists, footings, fasteners, structural members, or code-compliance details.
05Does the sketch show an exact cut plan?
No. It is only a sanity-check view of rows across the deck width and stock-board breaks along the deck length. Actual seam placement and cut optimization can differ on site.

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