How Do You Calculate Shingles for a Roof?

A simple, step-by-step guide with formulas, and worked examples (including waste factor)

Knowing how many shingles you need before ordering saves time and money. The basic workflow:

1. Measure the roof’s horizontal footprint (length × width).
2. Convert that to roof area by applying a slope (pitch) multiplier.
3. Convert roof area to squares (1 square = 100 sq ft).
4. Add a waste factor (10%–20% depending on roof complexity).
5. Convert squares to bundles (bundles per square depends on shingle type).
6. Add material for ridges, hips, and starter strips.

Below are the formulas, and examples.

1) Step 1 — Find the roof footprint (flat projection)

Formula:

$\text{Footprint}=L\times W$

Example: house length 40 ft, width 30 ft

Numerical:

$\text{Footprint}=40\times 30=1200{\text{ ft}}^2$

2) Step 2 — Apply the slope (pitch) multiplier to get the true roof area

Roofs are not flat — slope increases the actual material area. Use either the pitch ratio (rise/run) or roof angle.

Common formulas for slope multiplier:

Using pitch (rise over run, e.g., 6/12):

$\text{Slope multiplier}=\sqrt{1+{\left(\frac{\text{rise}}{\text{run}}\right)}^2}$

Example: pitch 6/12 (rise = 6, run = 12)
Numerical:

$M=\sqrt{1+{\left(\frac{6}{12}\right)}^2}=\sqrt{1+(0.5{)}^2}=\sqrt{1+0.25}=\sqrt{1.25}\approx 1.1180339887$

 

Using roof angle θ (degrees):

$M=\frac{1}{cos\left(\theta \right)}$

Now compute actual roof area:

Formula:

$\text{Roof area}=\text{Footprint}\times M$

Example (continuing): Footprint = 1200 ft², ($M\approx 1.1180339887$)

Numerical:

$A=1200\times 1.1180339887\approx 1341.6407864\text{}{\text{ft}}^2$

(Round sensibly for ordering — we’ll do that after waste factor.)

3) Step 3 — Convert roof area to roofing “squares”

Roofing is commonly ordered in squares:

1 square = 100 sq ft.

Formula:

$\text{Squares}=\frac{\text{Roof area}}{100}$

Example: ($A\approx 1341.6407864$) ft²

Numerical:

$S=\frac{1341.6407864}{100}=13.416407864\text{}\text{squares}$

4) Step 4 — Add waste factor (cut-offs, hips, valleys, starter strips)

Recommended waste factors:

• Simple gable roof (no hips/valleys): ~5%–10%
• Average roof (some hips, valleys, chimneys): ~10%–15%
• Complex roof (many hips, valleys, dormers): ~15%–20%

Formula:

$\text{Total squares with waste}=S\times (1+w)$

where (w) is the waste fraction (e.g., 0.10 for 10%).

Example: assume 10% waste (w = 0.10)

Numerical:

$S_{\text{total}}=13.416407864\times (1+0.10)=13.416407864\times 1.10=14.7580486504\text{}\text{squares}$

Always round up to the next whole square when ordering shingles (you can’t buy a fractional square). So order 15 squares in this example.

5) Step 5 — Convert squares to bundles (and to packs or boxes)

Shingles are sold in bundles. The number of bundles per square varies by shingle type:

• Typical 3-tab asphalt shingles: 3 bundles per square
• Typical architectural/laminate shingles: often 3 bundles per square (sometimes 3–4; check manufacturer)
• Specialty or heavy-duty shingles may differ; always check product spec.

Formula:

$\text{Bundles required}=S_{\text{total}}\times B$

where (B) = bundles per square.

Example: use (B=3) bundles/square and ($S_{\text{total}}\approx 14.7580486504$)

Numerical:

$B_{\text{needed}}=14.7580486504\times 3=44.2741459512\text{}\text{bundles}$

Round up to whole bundles — order 45 bundles.

6) Step 6 — Starter strip, ridge caps, hips, and valleys

These items are not included in standard bundle counts; calculate separately.

Starter strip

• Many roofers use a starter strip along the eaves. Starter strips are often cut from shingle bundles.
• A typical recommendation: set aside one bundle per 4–6 squares as starter strips (varies). For conservative ordering, include 1 bundle for starter on small roofs, more for large.

Ridge caps (hip & ridge)

• Ridge cap shingles run along the roof ridges/hips. Ridge length (linear ft) determines how many ridge-cap pieces you need.
• Manufacturer specs often state linear feet covered per bundle of ridge cap (e.g., 20 ft per bundle — check product).

Formula for ridge bundles

$\text{Ridge bundles}=\frac{\text{Total ridge length (ft)}}{\text{Coverage per bundle (ft)}}$

Example: For a simple gable roof, ridge length = 40 ft. If coverage per bundle = 20 ft:

$R=\frac{40}{20}=2\text{}\text{bundles}$

Valleys and complex areas

Valleys may require extra shingles and flashing. Add extra bundles depending on valley count — a cautious estimator adds 1–3 bundles for valleys on medium complexity roofs.

7) Full worked example (all together) — Simple gable roof

Given:

• House length (L = 40) ft
• House width (W = 30) ft
• Roof pitch (= 6/12)
• Waste factor (w = 10%) (0.10)
• Bundles per square (B = 3)
• Ridge coverage per bundle (= 20) ft

Step A — Footprint:

$\text{Footprint}=40\times 30=1200\text{}{\text{ft}}^2$

Step B — Slope multiplier: (6/12)

$M=\sqrt{1+{\left(\frac{6}{12}\right)}^2}=\sqrt{1.25}\approx 1.1180339887$

Step C — Roof area

$A=1200\times 1.1180339887\approx 1341.6407864\text{}{\text{ft}}^2$

Step D — Squares:

$S=\frac{1341.6407864}{100}=13.416407864\text{}\text{squares}$

Step E — Add 10% waste:

$S_{\text{total}}=13.416407864\times 1.10=14.7580486504\text{}\text{squares}$

Round up to whole squares: 15 squares.

Step F — Bundles (3 bundles/square):

$B_{\text{needed}}=14.7580486504\times 3=44.2741459512$

Round up to whole bundles: 45 bundles.

Step G — Ridge caps (40 ft ridge, 20 ft per bundle):

$R=\frac{40}{20}=2\text{}\text{ridge bundles}$

Order recommendation summary for this example:

• Shingle bundles: 45 bundles
• Ridge cap bundles: 2 bundles
• Starter strips: use material from shingles; reserve ~2–3 bundles or follow manufacturer guidance (we recommend 1–2 bundles set aside for starter depending on installer preference)
• Total shingles ordered: 45 bundles (plus ridge and starter as above)

8) Quick checklist & pro tips

• Always round up — you can’t buy fractions of a bundle. Extra shingles are useful for repairs later.
• Confirm bundles-per-square with the manufacturer (packaging will state coverage). Architectural shingles sometimes need 3 or 3.5 bundles/square.
• Choose waste factor by roof complexity: 5–10% simple, 10–15% average, 15–20% complex.
• Keep extras: store leftover bundles in a dry place — they match the batch and help future repairs.
• Don’t forget underlayment and flashing — these are measured separately (roll coverage, linear feet for flashing).
• Hire a pro for complex roofs (multiple valleys, hips, steep pitch) — estimating gets harder and mistakes are costly.

9) Handy formula summary

Footprint:

$\text{Footprint}=L\times W$

Slope multiplier (pitch):

$M=\sqrt{1+{\left(\frac{\text{rise}}{\text{run}}\right)}^2}$

Roof area:

$A=\text{Footprint}\times M$

Squares:

$S=\frac{A}{100}$

Squares with waste:

$S_{\text{total}}=S\times (1+w)$

Bundles required:

$B_{\text{needed}}=S_{\text{total}}\times B$

Ridge bundles:

$R=\frac{L_{\text{ridge}}}{C_{\text{ridge}}}$

Final notes

This method works well for simple roofs (gable, hip with few features). For roofs with many dormers, valleys, hips, or complicated geometry, measure each roof plane separately and consider using a professional estimator or roof-measuring software (these use the same formulas but handle complex geometry automatically).