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Free online gutter calculator

Gutters protect your foundation by channeling rainwater away from the house. Enter your eave length and roof area to get linear feet of gutter, downspout count, and all the.

About this calculator

How to use

  1. Enter your total roof area and eave length.
  2. Set your preferred downspout spacing (40 ft is standard).
  3. Choose gutter size and style.
  4. Adjust the waste factor if needed.
  5. Read linear footage and all fittings.

Formulas

gutterLinearFt        = eaveLength × (1 + waste/100)
downspoutsNeeded      = ceil(eaveLength / downspoutSpacing)
elbowsNeeded          = downspoutsNeeded × 2
endCapsNeeded         = 2  (one run assumption)
sections              = ceil(gutterLinearFt / 10)
jointConnectorsNeeded = max(0, sections − 1)

Capacity reference:
  4 in K-style → up to 5,000 sq ft roof
  5 in K-style → up to 8,000 sq ft
  6 in K-style → up to 12,000 sq ft

Worked example

1,200 sq ft roof, 100 lin ft of eave, 40-ft downspout spacing, 5-in K-style, 10% waste:

gutterLinearFt        = 100 × 1.10 = 110 ft
downspoutsNeeded      = ceil(100 / 40) = 3
elbowsNeeded          = 3 × 2 = 6
sections              = ceil(110 / 10) = 11
jointConnectorsNeeded = 11 − 1 = 10
recommendedSize       = 5in (1,200 ≤ 8,000)

Notes

  • Gutters should slope 1/4 inch per 10 feet toward the downspout; account for this when cutting sections.
  • Hidden hanger brackets every 24 to 36 inches hold gutters better than spike-and-ferrule in high-snow areas.
  • Seamless gutters eliminate most joint connectors — they are cut on-site by a professional gutter machine.

Frequently asked

How much gutter do I need?
Measure the total length of your eaves — gutters run along the eaves, not the rakes (sloped edges). Add a 10% waste factor for cuts and mitered corners.
How many downspouts do I need?
One downspout per 40 linear feet of gutter is a common rule of thumb. A 100-foot eave needs at least 3 downspouts. Also check capacity — a 5-inch K-style gutter handles roofs up to about 8,000 sq ft.
What size gutters should I choose?
4-inch K-style gutters handle roofs up to 5,000 sq ft. 5-inch handles up to 8,000 sq ft — the most common residential size. 6-inch gutters are for large roofs over 8,000 sq ft. The calculator recommends a size based on your roof area.
What is the difference between K-style and half-round gutters?
K-style gutters have a flat back and decorative ogee front — they hold more water and are easier to attach to fascia. Half-round gutters have a curved bottom, look more traditional, and are common on older homes. Flow capacity is similar for the same size.
How many elbows do I need?
Each downspout needs two elbows — one at the top where the downspout leaves the gutter and one at the bottom to direct water away from the foundation.
How do I share my gutter calculation?
Click Share for the page link or Share with my numbers to embed your roof and eave measurements in the URL.

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