Roofing & Decks Calculators
Roofs, decks and fences are where small math mistakes get expensive. Measure and convert roof pitch, turn a roof footprint and pitch into squares and bundles, turn deck dimensions into boards, joists and fasteners, and turn a fence line into posts, rails, pickets and bags of post concrete.
- Deck CalculatorHow many deck boards do you need? Count boards with gap spacing, joists at 12, 16 or 24 in on center, and deck screws for any rectangular deck.Open calculator
- Roofing CalculatorHow many bundles of shingles do you need? Turn a roof footprint and pitch into squares, bundles, ridge cap, drip edge and underlayment. Includes a pitch calculator.Open calculator
- Fence CalculatorCalculate fence posts, rails and pickets for privacy, board-on-board or split rail fences, plus bags of fence post concrete and gravel for every hole.Open calculator
- Roof Pitch CalculatorConvert roof pitch between rise and run, X in 12, degrees and percent slope. Get the pitch multiplier, hip and valley factor, and rafter length with overhang.Open calculator
Roofs: footprint × pitch
You don’t need to climb onto the roof to estimate shingles. Multiply the footprint (including overhangs) by the slope factor for the pitch to get the true roof area, then convert to squares of 100 sq ft. The roofing calculator does this and adds ridge cap, drip edge and underlayment, and the roof pitch calculator converts a measured rise and run, X/12 pitch, degrees or percent slope, and gives the rafter length.
Decks: boards, joists, fasteners
Deck surface materials follow simple geometry: rows of boards plus gaps across the width, joists at a fixed spacing along the length, and two screws at every crossing. The deck calculator counts all three. Framing sizes, spans, ledger attachment and footings are governed by local code (most US jurisdictions use IRC section R507).
Fences: posts, rails, pickets and post concrete
A fence is counted section by section: one post per section, one more to end each run and one per gate. Rails are two per section for a 4 ft fence and three for a 6 ft fence, and pickets are the length in inches divided by the board width plus the gap. Each post hole takes gravel and concrete: QUIKRETE’s rule is a hole three times the post width and ⅓–½ of the above-ground height deep plus 6 in of gravel. The fence calculator does all of it for privacy, board-on-board and split rail fences, including the bags of fence post concrete.
Safety first
Roof work above about 8/12 pitch, or any height you aren’t comfortable at, is a job for a pro with proper fall protection.
Slope factors for common pitches
The slope factor is √(1 + (rise ÷ 12)²). Multiply the footprint by it to get the area you will actually cover:
| Pitch | Slope factor |
|---|---|
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 12/12 | 1.414 |
A worked roof example
A 40 × 30 ft gable roof footprint at 6/12 pitch is 1,200 × 1.118 = 1,342 sq ft of roof, or 13.4 squares. Add 10% for a simple gable (more for hips and valleys) and you are at about 14.8 squares. Most 3-tab and architectural shingles come three bundles to a square, so that is about 45 bundles. Some heavier lines are packed four or five bundles per square, so check the wrapper before you order.
Deck boards and joist spacing
Joist spacing sets how many joists you need and limits which boards you can use. The International Residential Code allows 5/4 wood decking on joists up to 16 in on center when the boards run square to the joists, and less when they run on a diagonal; composite boards follow the spacing in the maker’s evaluation report. Rows of boards are the deck depth divided by the board width plus gap: a 12 ft deep deck with 5.5 in boards and 3/16 in gaps needs 26 rows. The American Wood Council’s free DCA 6 guide explains code-based framing, ledgers and footings in plain language.
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