Wet Room Fall Gradient Calculator

Estimate shower floor fall, slope ratio, drain height, threshold margin, and tile-to-former buildup for a wet room layout.

📌Wet room presets

📏Gradient inputs

Measurement units Preset values convert when you switch units.

Drain and fall layout This adjusts the wording and practical slope note.

Longest distance to drain (mm) Use the furthest tile edge or doorway point draining to the outlet.

Wet area width across fall (mm) Used for area and perimeter variation checks.

Target fall ratio Smaller ratio numbers create more drop.

Custom ratio denominator Example: enter 75 for a 1:75 fall.

Tray former or recessed base depth (mm) Common pre-formed boards are often around 20-30 mm before finishes.

Tile, adhesive, membrane buildup (mm) Add tile thickness plus adhesive bed and membrane allowance.

Available doorway threshold height (mm) Often checked against a low 10-15 mm level-access threshold target.

Main tile size along fall (mm) Shows the drop across one tile, useful for lippage planning.

Wet room gradient result

Vertical fall to drain 12.5 mm 0.49 in over 1.00 m

Gradient ratio 1:80 1.25% slope

Wet area coverage 0.90 m² 9.69 ft²

Threshold margin 2.5 mm Review splash/doorway detail

📊Calculated wet room markers

12.5

mm fall per metre

Single

fall plane style

3.8

mm drop per tile

mm total buildup

📐Reference tables

Fall ratio	Slope percent	Drop over 1 m	Typical wet room use
1:100	1.00%	10.0 mm	Gentle background floor fall outside the main shower spray zone
1:80	1.25%	12.5 mm	Common wet room target when balancing drainage and comfortable standing
1:60	1.67%	16.7 mm	Stronger shower fall for longer runs or higher water volume areas
1:50	2.00%	20.0 mm	Steeper fall often used for compact shower zones and fast drainage
1:40	2.50%	25.0 mm	Very noticeable slope, usually reserved for short runs or special layouts

Drain distance	Fall at 1:80	Fall at 1:60	Fall at 1:50
600 mm	7.5 mm	10.0 mm	12.0 mm
750 mm	9.4 mm	12.5 mm	15.0 mm
900 mm	11.3 mm	15.0 mm	18.0 mm
1200 mm	15.0 mm	20.0 mm	24.0 mm
1500 mm	18.8 mm	25.0 mm	30.0 mm

Threshold height	Common interpretation	Fall margin at 1:80	Notes for wet rooms
0 mm	Flush transition	No retained height	Needs careful containment because there is no spare vertical edge
5 mm	Very low lip	Covers 400 mm run	Useful for small shower entries where the drain is close
10 mm	Low threshold	Covers 800 mm run	Often enough for compact one-way falls to a nearby linear drain
15 mm	Accessible max target	Covers 1200 mm run	Common planning reference for level-access doorways
20 mm+	Raised step	Covers 1600 mm+ run	More containment, but less like a flush wet room transition

Layer or former	Typical thickness	Calculator field	Planning use
Tile adhesive bed	3-6 mm	Buildup	Thin-bed adhesive allowance under porcelain or ceramic tile
Floor tile	8-12 mm	Buildup	Main finish thickness before grout and edge trim selection
Sheet membrane	0.5-1.5 mm	Buildup	Waterproofing layer often counted inside finish buildup
Tile backer board	6-12 mm	Former/base	Support board or overlay layer above an existing floor
Pre-sloped tray former	20-30 mm	Former/base	Factory-sloped wet room base checked against finished floor height

💡Calculation notes

Drain run: measure the longest path water must travel to the drain, not just the shower tray length.

Threshold: compare the doorway upstand with the calculated vertical fall before choosing a flush or raised entry detail.

Tile format: large tiles can bridge slope changes; smaller mosaics are easier around point drains and compound falls.

Former depth: add the former, tile, adhesive, and membrane so the finished wet room floor can be matched to nearby flooring.

The most important thing about a wet room for good drainage is the slope of the floor, because water will find its way onto level surfaces and then into places you don’t want it. Even with great drains and great tiles, they is no use unless you calculate the slope exactly right to make sure that the water will actualy flow to the drain. Instead of thinking about where the drain goes, you should thinks about how far the water has to travel to get there so that water does not pool.

When you enter the length of the longest run and the amount of slope you want, the calculator do all that math for you and eliminates any potential human error involved with conversions. In the past, people used to think they could measure the width of their space, which would lead them to forget about those corners. Corners or diagonals always gets wet.

How to Measure the Floor Slope for a Wet Room

So instead of measuring from the middle of the room, what you have to do is take the longest distance from the drain outlet to the outside edge of the wet area (that’s normaly where most people go wrong). In my example, that long corner was 1200mm away from the drain. At that measurement, if I wanted a typical one to eighty, the minimum fall needed to properly direct the water flow would be fifteen millimetres. That isn’t much at all, but it makes a huge difference between having a dry floor or a dangerous slippery wet one.

To pick the right slope ratio, consider both user comfort and how fast water drains: A perfectly smooth surface of no more than a one to hundred fall rate is invisible underfoot but works only if the surface is perfectly smooth, whereas a steeper rise (like a one to sixty or eighty ratio). Is the sweet spot for most residential shower without being uncomfortable on bare feet. In practice, a one to eighty or one to sixty ratio is usually ideal for a shower (shown above), which leaves just enough velocity to remove splash water promptly without making the incline too extreme.

You’ll note from the calculator that it tells you precisely what vertical fall is needed for any given set of room sizes; and whether this exceeds your available doorway threshold. If so, change the plan or live with a steeper slope right away. However, many designs are also constrained by threshold height: you want a level access door but some kind of barrier separating the wet and dry zones. The tool then checks the measured fall from the floor in front of the door to the point where it drains away. It checks whether this leaves sufficient margin over the threshold. If it doesn’t, i.e., the negative margin; water will overflow the threshold before it reaches the drain, so that’s a red flag.

To solve it, you may have to raise the threshold slightly, perhaps with a small three millimeter step, or relocate the drain closer to the door. And the reference tables explains how to get threshold heights matching different run lengths, including why fifteen millimeters is a popular choice for an accessible design over a one meter two hundred run.

Remember to include the cumulative build-up of the floor layers too (the floor doesn’t begin life as zero). That includes structural subfloor, pre-sloped base or tray former, adhesive bed, waterproofing membrane and tiled surface. This adds up to a total buildup number. This number lets you calculate whether your finished floor might be higher than the surrounding bathroom floor, which could become a tripping hazard if you neglect the height. To help you match the edge of your tile to other floors or the door frame, it calculates a cumulative build-up figure.

For example, large format tiles are attractive, but they can conceals any slope flaws. Smaller mosaics show more of the slope and let the adhesive bed flex. In a wet room it’s the slope that makes the promise of ‘water behaving correctly’ come true, and it’s the shape beneath the pretty surfaces that make this work. Respect the total buildup so that the slope can do its job effectively and check the longest run.

You should of gotten the math right and the water drains away efficiently and the floor stays safe.

Wet Room Fall Gradient Calculator