Gutter Sizing – Catchment & Downspouts

This tool applies the formula Gutter Size = f(Plan Area): 45 m² → Ø100; 100 m² → K125; Downspouts = Area ÷ 50 directly to the measurements you enter, so the result is an exact quantity rather than a rough guess. Because gutters is priced and delivered per unit - cubic meters, square meters, bags or tonnes - a small measurement error multiplies across the entire order, which is why the calculation is worth doing properly. Worked example: A 120 m² plan roof needs a 125 mm K gutter and 2-3 downspouts (120 ÷ 50 = 2.4). One note from practice: One downspout per 12 m of gutter keeps water moving even in cloudbursts. Enter the roof plan area and rainfall zone, check the totals, then round up to the nearest whole bag, sheet or roll before you place the order.

1. 1

   Take dimensions

   Note the roof plan area and rainfall zone for the area you are covering or filling.

2. 2

   Run the calculation

   The calculator applies Gutter Size = f(Plan Area): 45 m² → Ø100; 100 m² → K125; Downspouts = Area ÷ 50 the moment you type.

3. 3

   Check the breakdown

   Compare base quantity and waste-adjusted totals side by side.

4. 4

   Plan the purchase

   Use the final figure to price and schedule the delivery.

• Forgetting underlayment

• Mismatched bundle counts

• Steep-slope hazards