Pergola Calculator
Plan the frame of a pergola before you buy lumber. Enter the width and depth, choose how many posts, and see rafters, slats, beam length and concrete bags for the footings.
| Detail | Result |
|---|---|
| Rafter length | 12 ft |
| Beam length (2 beams) | 14 ft |
| 60 lb concrete bags | 16 |
| Total lumber, all parts | 334 ft |
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How Pergola Calculator Works
Four quick steps happen behind the screen every time you change a number.
Spaces the rafters
The width is divided by your rafter spacing, and one more is added to close the last gap.
Cuts rafters to length
Each rafter spans the depth plus the overhang on both ends.
Counts top slats
Slats run across the rafters at the spacing you choose and shade the space below.
Sizes the footings
Footing diameter and depth give the concrete volume per post, shown as 60 lb bags.
How to Use Pergola Calculator
Five quick steps take you from a tape measure to a usable result.
- Set the footprintEnter the width and depth you want to shade, such as 12 by 10 feet.
- Choose the post heightEight feet leaves room to walk under the beams.
- Pick the number of postsFour posts suit small frames. Choose six or eight for spans over 12 feet.
- Set rafter spacingSixteen inches is a strong, common choice. Twenty four inches uses fewer boards.
- Read the lumber listTake the counts and lengths to your supplier and add the concrete bags.
Formula Used in Pergola Calculator
These are the standard formulas behind every result, so you can check the numbers yourself.
Rafters
The ⌊ ⌋ marks mean round down, then add one end rafter.
Rafter length
D is the depth of the pergola in feet.
Footing concrete
r is the footing radius, and a 60 lb bag yields about 0.45 cubic feet.
Worked example
A 12 ft by 10 ft pergola with rafters every 16 inches needs 10 rafters. Each rafter is 10 + 2 = 12 ft long. Four 10 inch footings 30 inches deep use 4 bags of concrete each, so 16 bags in all.
Pergola Rafter Chart
Rafters needed across a 12 ft wide pergola at each spacing.
| Size | Rafters | Rafter length |
|---|---|---|
| 12 inch spacing | 13 | 12 |
| 16 inch spacing | 10 | 12 |
| 24 inch spacing | 7 | 12 |
Rafter length includes a 1 ft overhang at each end of a 10 ft deep pergola.
Pergola Lumber by Size
Rafters, slats and concrete for common pergola sizes with four posts, 16 inch rafters and 12 inch slats.
| Size | Rafters | Top slats | Lumber, ft |
|---|---|---|---|
| 8 × 8 ft | 7 | 9 | 212 |
| 10 × 10 ft | 8 | 11 | 284 |
| 12 × 10 ft | 10 | 11 | 334 |
| 12 × 12 ft | 10 | 13 | 382 |
| 14 × 12 ft | 11 | 13 | 426 |
| 16 × 12 ft | 13 | 13 | 484 |
Lumber totals add rafters, two beams, slats and posts.
Common Pergola Mistakes
Four slips that throw off the result, and how to avoid them.
Spanning too far
Long beams sag. Add a middle pair of posts when the width goes past 12 feet.
Setting posts too shallow
Shallow footings let the frame rock in wind. Dig below the frost line in cold areas.
Forgetting the overhang
Rafters and beams run past the posts. Without the overhang, the lumber list falls short.
Skipping the permit check
Many places require a permit for a fixed structure. Check local rules before you dig.
Tools You Must Know
One number is rarely enough for a yard project. These calculators pair well with this one.
Pergola Calculator FAQs
Quick answers before you order materials.
How many rafters does a pergola need?
Divide the width in inches by the spacing and add one. A 12 ft pergola with 16 inch spacing needs 10 rafters.
How far apart should pergola posts be?
Keep posts no more than 10 to 12 feet apart so the beams do not sag.
How deep should pergola footings be?
Most designs use 30 inches or more, and below the frost line in cold climates.
What size lumber is used for a pergola?
Posts are often 6 by 6 inch, beams 2 by 8 inch pairs, and rafters 2 by 6 inch boards.
How many bags of concrete per post?
A 10 inch wide, 30 inch deep footing needs about 3 to 4 bags of 60 lb concrete.
Is the Pergola Calculator free?
Yes. It is free to use, with feet or metres and no signup.
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