Patio Fall Calculator: Slope & Leveling Guide

A patio fall calculator works out the slope you need on a patio surface so rainwater drains away instead of pooling. You enter the patio length and the fall you want (usually 1:80, meaning 1 unit of drop per 80 units of length), and it returns the total drop in inches or millimeters, plus the slope angle in degrees. This tells you exactly how much lower one edge of the patio must be than the other.

Formula

The patio fall is a simple ratio. For a fall of 1 in 80, every 80 units of horizontal length drops by 1 unit. The formulas used here are:

• Total drop (inches) = (patio length in feet × 12) ÷ fall ratio
• Total drop (millimeters) = (patio length in feet × 304.8) ÷ fall ratio
• Slope angle (degrees) = arctan(1 ÷ fall ratio)

Where:

• Patio length (l) is the distance from the high edge to the low edge, measured in feet.
• Fall ratio (r) is the horizontal distance per unit of drop. A ratio of 80 means 1 inch drop per 80 inches of run.
• Drop is the vertical difference between the two edges, in inches or millimeters.
• Angle is the slope expressed in degrees, useful if you are setting a level or laser.

Worked Example

Using the default values: a patio 12 feet long with a 1:80 fall.

1. Convert length to inches: 12 ft × 12 = 144 inches.
2. Divide by the ratio: 144 ÷ 80 = 1.80 inches.
3. Convert to millimeters: 12 ft × 304.8 = 3657.6 mm; 3657.6 ÷ 80 = 45.72 mm.
4. Angle: arctan(1/80) = arctan(0.0125) ≈ 0.72°.

The calculator returns a drop of 1.80 inches (45.7 mm) and an angle of 0.72°. That matches the manual calculation.

Why Patio Fall Matters

Standing water on a patio is more than a nuisance. Over time, water seeps into joints between paving slabs, washes out sand or mortar, and can undermine the base layer. In freezing climates, trapped water expands when it turns to ice, lifting or cracking pavers. A properly calculated fall moves water off the surface quickly, protecting the patio and the area next to your house. Most paving contractors aim for a minimum fall of 1:80, which is roughly a 1.25 cm drop per meter. Some use 1:60 for larger areas or where heavy rain is common, and 1:40 for very smooth surfaces like polished concrete that drain more slowly.

How to Measure and Set the Fall

Once you have the total drop from the calculator, you need to transfer that measurement to the ground. Here is the practical process:

1. Mark the high edge against the house or the highest point of the patio.
2. Drive a nail or screw into the top of the formwork at the high edge.
3. Measure the calculated drop down from that nail at the low edge and mark a second nail there.
4. Stretch a string line between the two nails. That string now represents the finished surface slope.
5. Use a spirit level on top of the string to check it is straight, then screed the base or mortar to that line.

For small patios, a long spirit level with a shim under one end works. For larger areas, a laser level with a slope function is faster and more accurate. Always check the fall in two directions if the patio drains to a corner.

Common Mistakes and Misconceptions

The most frequent error is mixing up the ratio. A fall of 1:80 means 1 unit down per 80 units across. Some people reverse it and cut 80 units down per 1 unit across, which creates a steep ramp. Another common mistake is measuring the fall over the diagonal instead of the true drainage direction. If your patio is 12 ft wide and 20 ft long, and you want water to run the 12 ft direction, use 12 ft in the calculator, not the diagonal of 23 ft. A third issue is ignoring the sub-base. The fall must be built into the compacted sub-base, not just the top few centimeters of mortar. If the base is level, you will end up with a thin edge at one side and a thick one at the other, which can crack.

A widespread misconception is that a steeper fall is always better. Falls steeper than about 1:40 become uncomfortable to walk on, especially with outdoor furniture, and can make chairs rock. Falls gentler than 1:100 may not drain fast enough during a downpour, allowing water to sheet across the surface. The 1:80 standard is a good balance for most patios.

Comparing Fall Ratios

Different surfaces and situations call for different falls:

• 1:40 (2.5%) – smooth concrete, large paved areas, or patios under overhangs where rain is less of an issue but washing down is common.
• 1:60 (1.67%) – a common choice for block paving and larger patios.
• 1:80 (1.25%) – the standard minimum for most paving slabs and natural stone.
• 1:100 (1%) – only for very textured surfaces with good drainage joints, and only in low-rainfall areas.

For reference, a 1:80 fall is about 0.72 degrees. A 1:40 fall is about 1.43 degrees. Most people cannot perceive a slope under 1 degree when walking on it, so a correct fall is invisible in daily use.

When the Calculator Does Not Apply

This calculator assumes a single, straight slope from one edge to the other. Some patios are designed with a fall to a central drain or a fall in two directions. For those, you need to calculate each segment separately, using the length of that segment as the input. The calculator also assumes the ground under the patio is already compacted and stable. If you are building on clay soil that expands when wet, or on a slope that needs retaining, the fall calculation is only one part of the design. You also need to consider drainage at the low edge — where will the water go once it runs off? If the low edge is against a wall or fence, you need a channel drain or a gravel trench to carry the water away.

Cost and Material Considerations

The fall itself does not add material cost, but achieving it correctly affects how much base material you need. If you calculate a drop of 2 inches across a 12 ft patio, the sub-base at the low edge is 2 inches thicker than at the high edge. That extra volume is usually small — for a 12×10 ft patio, a 2-inch average extra across half the area is about 0.37 cubic yards of material. In practice, most contractors build the sub-base slightly above the finished level and screed down to the string line, so the extra material is minimal. The real cost is in labor and the leveling tools. A laser level with a slope function costs more than a basic spirit level but saves time on larger patios.

Safety and Accuracy Notes

Always check the fall before the mortar or concrete sets. Once the surface hardens, correcting a wrong fall means breaking out and re-pouring. For concrete patios, the fall should be checked with a straightedge and level while the concrete is still workable. For paving slabs, check every few slabs as you lay them. The angle output from this calculator is useful for setting a digital inclinometer, but most spirit levels do not show fractions of a degree accurately. Use the drop measurement in inches or millimeters for setting string lines, and use the angle only as a cross-check. If you are working near a house foundation, ensure the fall directs water away from the building, never toward it. Building regulations in many areas require a minimum fall away from the dwelling, so confirm local requirements before starting.

Frequently Asked Questions

What is the standard fall for a patio?

The most common fall is 1:80, which means for every 80 inches of patio length, the surface drops 1 inch. This equals about 12.5 mm per meter. Many contractors use 1:60 for larger areas or where heavy rain is common.

How do I calculate the fall for a patio in millimeters?

Measure the patio length in meters, then divide by the fall ratio. For a 1:80 fall on a 4-meter patio, the drop is 4 ÷ 80 = 0.05 meters, which is 50 mm. The calculator above does this automatically when you enter the length in feet.

Can a patio have too much fall?

Yes. Falls steeper than about 1:40 become uncomfortable to walk on, especially when wet, and can cause furniture to rock. A very steep fall also makes the high edge noticeably higher than the surrounding ground, which can look odd and create a trip hazard.

Should the fall go away from the house?

Yes, always. The patio surface should slope away from the house foundation so rainwater runs off and does not pool against the wall. If the patio is not attached to the house, the fall should still direct water to a drainage point, not toward the building.

How do I check the fall after the patio is laid?

Place a long spirit level on the surface parallel to the fall direction. Put a shim under the downhill end until the bubble is centered. Measure the shim thickness and divide by the distance between the level ends. For example, a 3 mm shim under a 1-meter level is a 1:333 fall, which is too gentle.

What if my patio drains to a corner?

Calculate the fall for each edge separately. For a corner drain, the surface must slope in two directions, so you need a fall along both the length and the width. Use the calculator twice, once for each dimension, and set the screed to the diagonal plane that connects the high corner to the low corner.

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in l | Patio length | ft | 12 | 0 | 1000
in r | Fall ratio (1 in X) | - | 80 | 10 | 200
out drop_in | Total drop | in | l * 12 / r | 2
out drop_mm | Total drop | mm | l * 304.8 / r | 1
out angle | Slope angle | deg | atand(1 / r) | 2