How to use this calculator
- Enter patio length. The dimension running away from the house (or away from any structure water shouldn't reach). Typical 8–20 ft.
- Enter slope %. Default 2% (≈ 1/4" per ft). Use 1.5% for very smooth surfaces, 2.5–3% if you want extra drainage in high-rainfall zones.
- Read the drop. The primary result is total drop in inches. Use this to set the form heights when pouring.
- Use slope ratio. For string-line setting: the calculator gives you the equivalent rise/run ratio (e.g. 1 in 50).
Formula
Drop (in) = Length (ft) × Pitch (% / 100) × 12
Worked example
A 12 ft patio at 2% slope: 12 × 0.02 × 12 = 2.88 inches total drop, or about 0.24" per foot. Set the high end of your forms 2.88" higher than the low end (the side away from the house).
Common project sizes
Quick reference for the most common concrete patio slope calculator use cases. Use these as a sanity check on your calculator inputs.
| Project | Dimensions | Result |
|---|---|---|
| 8-ft patio at 2% | 8 ft × 2% slope | 1.92" total drop |
| 12-ft patio at 2% | 12 ft × 2% slope | 2.88" total drop |
| 16-ft patio at 2% | 16 ft × 2% slope | 3.84" total drop |
| 20-ft patio at 2.5% | 20 ft × 2.5% slope | 6" total drop |
How we calculate this
Assumptions baked in
Drop over length = length × (pitch% / 100). Standard residential drainage slope is 1/4" per ft, which equals 2.08% (or 1:48 ratio). The minimum to shed water is 1% (1/8" per ft). The maximum before a patio surface becomes uncomfortable to set furniture on is 3%.
Accuracy and margin of error
Math is exact. The error comes from setting the forms to the calculated drop — use a 4 ft mason's level with a 1" block under one end as a quick "draftsman's slope" reference, then double-check with a string-line and tape measure.
Edge cases this calculator does not handle
Pour slope AWAY from the house — minimum 6" of fall in the first 10 ft, per IRC R401.3. For an entry patio that ties to a sliding door threshold, the high side of the patio sits 1" below the door threshold (and the slope goes from there). For pool decks, slope away from the pool coping to avoid scum and chemicals running back into the water.
Cited sources for this page
The figures and rules above are anchored to the following normative references. We link the underlying claim to its standard — not as generic SEO trust signals, but so you can audit any number on this page against a primary source.
Residential exterior drainage slope of 1/4 inch per foot (2.08%) minimum away from foundation is specified in IRC R401.3.
Tips for accurate results
- Always slope away from the house — minimum 2% (1/4" per ft). Never slope toward a building, even a shed.
- For large patios (20+ ft), consider sloping toward a center channel drain instead of edge runoff. Easier to manage standing water.
- A 4% (1/2" per ft) slope is the maximum that still feels level when walking. Anything steeper feels sloped underfoot.
- Use a string line + line level to set forms; don't rely on a 4-ft level — the 1/4" per ft drop is invisible at that distance.
- The slope is set during forming, not pouring. Frequency of mistakes: forms set level "to look right", then drainage fails after the first rainstorm.
When to consult a pro
Slope-setting is one of the steps where DIY pours go wrong most often. The drop is invisible to the naked eye, so even careful workers set forms level. Hire a pro for any patio over 200 ft² — they have the screed boards and string-line techniques to maintain consistent slope across the surface, and the experience to spot a back-pitched form before the pour.