Pergola Rafter Count

Same math as counting fence posts — the number of gaps is width over spacing, but rafters sit at both ends too, so add one.

Your numbers

Roof width(m)Rafter span(m)Rafter spacing(m)

Rafters needed

Total lumber length

40m

What it means

10 rafters at 4 m each — buy 40 m of lumber in total.

Formula

Rafters needed = Roof width ÷ Rafter spacing + 1

If you plan to overhang the eaves for extra shade, add that overhang to the rafter span before entering it.

What to enter

Input	Default	Accepted range
Roof width (m)	3.6	0.5 and up
Rafter span (m)	4	0.5 and up
Rafter spacing (m)	0.4	0.1 and up

Step by step

FormulaRafters needed = Roof width ÷ Rafter spacing + 1

With the default numbersRafters needed = 3.6 ÷ 0.4 + 1

AnswerRafters needed = 10

Quick reference table

Results when only Roof width (m) changes and everything else stays put.

Roof width (m)	Rafters needed	Total lumber length (m)
1.8	6	24
2.7	8	32
3.6	10	40
5.4	15	60
7.2	19	76

What each result means

Result	At default values
Rafters neededSame math as fence posts: the gap count plus one.	10
Total lumber length (m)Rafter count times the length of one — the total lumber to order.	40

Common mistakes

If you plan to overhang the eaves for extra shade, add that overhang to the rafter span before entering it.

Glossary

Rafters needed: Same math as fence posts: the gap count plus one.
Total lumber length: Rafter count times the length of one — the total lumber to order.

Frequently asked questions

QHow is Pergola Rafter Count calculated?

Rafters needed = Roof width ÷ Rafter spacing + 1 — Same math as counting fence posts — the number of gaps is width over spacing, but rafters sit at both ends too, so add one.

QCan you walk through an example?

With Roof width 3.6m, Rafter span 4m, Rafter spacing 0.4m, the answer is Rafters needed 10.

QWhat do I need to enter?

Enter Roof width, Rafter span, Rafter spacing. The result recalculates as you type, and an empty box counts as zero.

QHow much does the answer move if I change a number?

Changing only Roof width (m) moves the answer to Roof width (m) 1.8 → Rafters needed 6 and Roof width (m) 7.2 → Rafters needed 19. The table below lays out five steps.

QHow are the numbers rounded?

Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.

QAnything to watch out for?

If you plan to overhang the eaves for extra shade, add that overhang to the rafter span before entering it.

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Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.