Calculate horizontal run or vertical drop using a fixed 1/4-inch-per-foot assumption. Choose the value to solve and the result unit.
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Deck Slope Formula
The calculator uses a fixed deck slope of 1/4 inch of drop per 1 foot of run. It can calculate either the total drop from a known deck length or the deck length from a known total drop.
- Dropin = total vertical fall, in inches
- Runft = total horizontal deck length, in feet
- 0.25 = slope rate, equal to 1/4 inch per foot
If you enter the total deck length, the calculator converts that length to feet and multiplies it by 0.25 to find the total drop in inches. If you enter the total drop, the calculator converts the drop to inches and divides it by 0.25 to find the required deck run in feet.
The unit menus only affect conversion. The slope itself stays fixed at 1/4 inch per foot.
Common Deck Drop Amounts at 1/4 Inch per Foot
Use this table to check typical results for common deck lengths.
| Deck Run | Total Drop | Total Drop |
|---|---|---|
| 4 ft | 1 in | 2.54 cm |
| 8 ft | 2 in | 5.08 cm |
| 12 ft | 3 in | 7.62 cm |
| 16 ft | 4 in | 10.16 cm |
| 20 ft | 5 in | 12.70 cm |
Deck Slope Equivalent Values
| Slope Description | Equivalent Value |
|---|---|
| Inches per foot | 1/4 in per ft |
| Decimal inches per foot | 0.25 in per ft |
| Approximate percent slope | 2.08% |
| Millimeters per meter | about 20.8 mm per m |
Example Calculations
Example 1: Find the total drop for a 12 ft deck
Use the drop formula:
The total drop is 3 inches.
Example 2: Find the deck run from a 4 inch drop
Use the run formula:
The deck run is 16 feet.
FAQ
Is 1/4 inch per foot the correct slope for every deck?
No. A 1/4 inch per foot slope is a common drainage slope, especially where water needs to move away from a structure. However, the correct slope can depend on the deck surface, waterproofing system, local code, and manufacturer instructions. Use the calculated value as a planning number, then confirm the required slope for your specific deck assembly.
Should deck length mean the actual board length or the horizontal run?
Use the horizontal run in the direction water should drain. For example, if water drains away from the house across a 10 ft deck depth, enter 10 ft. Do not use the diagonal distance across the deck unless the slope actually follows that diagonal path.
How do you choose the value to calculate?
Select vertical drop and enter a positive horizontal run, or select horizontal run and enter a positive vertical drop. The calculator uses the fixed 1/4 inch per foot assumption in either direction.
