Calculated result
18pairs
Rafter pairs to plan
Formula
LUM-RAFTER-001Rafter length = √((width ÷ 2)² + ((width ÷ 2) × rise ÷ 12)²)
36 rafters
Individual rafters
11.18 ft
Rafter length before cuts
402.49 ft
Planning linear footage
Lumber & framing calculator
Rafter calculator
Preliminary paired-rafter count and rafter length from roof plan dimensions, pitch and spacing.
By Feroz Sheikh · Formula LUM-RAFTER-001 v1.0.0 · Tested formula
Formula
LUM-RAFTER-001v1.0.0Project inputs
Enter your measurements. Switching units converts the same project, including coverage and unit rates. Package yields keep the unit printed beside each field.
Refine measurements and rates at any time. Your current values are included in the material-list PDF.
Planning estimate, not engineering advice. Confirm site measurements and product specifications. Calculation limits.
This calculator uses roof width, rise-per-12 pitch and stated spacing to produce a preliminary paired-rafter count and sloped rafter length.
- Measure building length and total horizontal building width.
- Enter roof rise per 12 and spacing specified by the project.
- Verify rafter size, birdsmouth, overhang, ridge detail, spans and connection requirements independently.
Add your project costs
Enter supplier prices and any labour, equipment or delivery costs. Pick a currency — badges, inputs, and totals follow.
Costs use the stick or sheet rate you type. Construclytic does not fetch lumber-yard prices. Hangers, hardware and labour stay companion lines.
How this calculator works
A transparent rafter calculator takeoff.
Feroz SheikhWritten by Feroz Sheikh · founder
This calculator uses roof width, rise-per-12 pitch and stated spacing to produce a preliminary paired-rafter count and sloped rafter length.
LUM-RAFTER-001 pairs rafters from plan width, pitch and spacing, then a rafter length. It does not cut birds-mouths or size a ridge. Hips and valleys are extra members.
Worked example. Example: a 30 ft building length at 24 in spacing has 16 rafter pairs. A 20 ft building width at 6:12 creates a 11.18 ft run per rafter before waste allowance.
See the example inputs and calculated result
This separate example uses the calculator's starting values in imperial units. It stays unchanged when you edit the tool above. Example rates, yields and dimensions are assumptions, not recommendations for your project.
| Input | Example value |
|---|---|
| Building length | 30 ft |
| Building width | 20 ft |
| Pitch rise | 6 rise per 12 |
| Rafter spacing | 24 in |
| Waste factor | 10 % |
Example result: Rafter pairs to plan — 18 pairs.
Rafter length = √((width ÷ 2)² + ((width ÷ 2) × rise ÷ 12)²)
Use the labelled units above when reproducing this example. To work in metric, switch the calculator to Metric; applicable measurements are converted, while product-specific units remain labelled.
- 1. Measure building length and total horizontal building width.
- 2. Enter roof rise per 12 and spacing specified by the project.
- 3. Verify rafter size, birdsmouth, overhang, ridge detail, spans and connection requirements independently.
Field notes that change the number
- Pitch is rise per 12. An 8:12 pair is longer than a 4:12 pair on the same span.
- Spacing is on-center. 16 in and 24 in do not share a count.
- Ridge, collar ties and hangers stay off this line.
Common mistakes
- Using plan half-span as rafter length on a steep pitch.
- Ignoring gable overhangs.
- Treating this as a truss design.
Need the broader workflow? Read the lumber & framing planning guide.
Questions
Questions about rafter calculator
Does this design a rafter?
No. It is a geometric quantity estimate only and cannot determine structural rafter sizing or connections.
Why show rafter pairs?
A simple symmetrical roof uses two rafters per spacing location; complex roofs must be taken off by their actual geometry.
What is the formula ID?
This rafter calculator publishes formula LUM-RAFTER-001, version 1.0.0. The ID stays on the result and on the estimate line.