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Digital level for a fall

Updated: October 2026

You are setting a waste pipe, a wet-room floor, or a short ramp, and a centred bubble only tells you the pipe is level. Level is the wrong target. A digital level for a fall reads millimetres per metre, or a 1:N figure, on a straight edge long enough for the run. Button-by-button zeroing stays on the [how to use a digital spirit level](/hub/measuring-equipment/digital-spirit-level/sub/digital-spirit-level-how-to-use) page. The twelve listings are on the [digital spirit level comparison](/hub/measuring-equipment/digital-spirit-level). Peter F. used Approved Document H and the amazon.co.uk listings. We did not design your drain. A qualifying purchase can earn a commission at no extra cost to you. The level does not approve Building Regulations work.

You are setting a waste pipe, a wet-room floor, or a short ramp, and a centred bubble only tells you the pipe is level. Level is the wrong target.

Our top picks

Mecurate Mecurate 15.7-inch digital level and angle finder

Best overall

Mecurate Mecurate 15.7-inch digital level and angle finder
9.0/10about £35.99
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Prexiso Prexiso PTD180 digital angle gauge

Preis-Tipp

Prexiso Prexiso PTD180 digital angle gauge
9.0/10about £29.99
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Huepar Huepar DL24 600 mm digital spirit level

Alternative

Huepar Huepar DL24 600 mm digital spirit level
7.8/10about £59.99
View offer
On this page
  1. 1.What a fall is in mm/m
  2. 2.Foul drains and Approved Document H
  3. 3.How to read it on the level
  4. 4.Showers, ramps, and when to stop

What a fall is in mm/m

A fall of 1:N means the drop is the horizontal run divided by N. At 1:40 the drop is 25 mm per metre. At 1:80 it is 12.5 mm per metre. Over 6 m, 1:40 is 150 mm of fall. Over a 1.2 m level, 1:40 is 30 mm of drop from end to end. Degrees will not show 25. By arithmetic, 25 mm per metre is about 1.43°. Switch to mm/m, or check the drop with a tape. A tenth of a degree is about 1.75 mm per metre, a real slice of a 12.5 mm fall. A 60 mm cube cannot represent a 6 m run. Length comes before the brand on the digital spirit level buying guide.

Foul drains and Approved Document H

Approved Document H (2015) Table 6 recommends minimum gradients for foul drains. For a 100 mm pipe, it shows 1:40 where peak flow is under 1 litre per second. Where flow is above 1 litre per second and at least one WC is connected, the minimum shown is 1:80. Say recommended minimum in that table. Do not say every 110 mm drain must be 1:40. Rainwater is a different table. A merchant’s recommended gradient is not that table. A pipe flatter than the minimum for its flow and diameter is the blockage risk the table is trying to avoid. A much steeper pipe can still be wrong for the fittings and the depth, which is a design question. If the drain is notifiable, or you are altering a shared run, stop. A confident mm/m reading does not notify Building Control.
Mecurate Mecurate 15.7-inch digital level and angle finder

Testsieger

Mecurate Mecurate 15.7-inch digital level and angle finder
9.0/10about £35.99
View offer

How to read it on the level

Set the mode to mm/m before you look for 25 or 12.5. About 25 mm/m is 1:40. About 12.5 mm/m is 1:80. Listings here that state mm/m include the Mecurate, the Huepar, the Prexiso, the SAVSEC, and the eSynic WT504. The Franklin only lights green. The Prexiso is 175 mm and cannot stand in for 6 m of pipe. A high spot between two samples will hold water even if both say 25. The spacer method packs one end so a 1.2 m level reads level when the pipe has 30 mm of drop, which is 1:40 over that length. The wrong end makes an uphill pipe look perfect. A digital bar in mm/m is harder to reverse if you know which way the pipe drops. Either method fails if the straight edge is too short.

Showers, ramps, and when to stop

A wet-room floor is not Table 6. Trade sites cite about 1:80 to 1:35. That is their citation, not a sentence from BS 5385. The tray and the tanking system decide the fall. Check their number in mm/m, on a bar that reaches the area. A lamp that only says level cannot. Too little fall ponds water. Too much is a trip. Approved Document M Table 1 limits a ramp flight: 1:12 for a going of 2 m, 1:15 for 5 m, and 1:20 for 10 m. Those are maximums, so steeper is worse, the opposite of a drain minimum. Width, landings, and handrails sit outside a spirit-level job. If you only came to choose a vial or a screen, the vial will not name the slope. When the work is notifiable, stop. Building Control outranks a £36 bar.

Common questions

What is 1:40 and 1:80 in millimetres per metre?
1:40 is 25 mm per metre. 1:80 is 12.5 mm per metre. The fall is the run divided by N, so 6 m at 1:40 is 150 mm of drop. A 1.2 m level at 1:40 is 30 mm from end to end. In degrees, 25 mm/m is about 1.43°. To see 25, the unit has to be mm/m.
Which minimum applies to a 100 mm foul drain with a WC?
Look at Approved Document H Table 6. For a 100 mm pipe above 1 litre per second, with at least one WC, the recommended minimum is 1:80. Below 1 litre per second the table shows 1:40 for 75 mm and 100 mm pipes. Do not apply 1:40 to every 110 mm drain. Rainwater is a different case. If you are unsure of the flow, ask a plumber before you bed the pipe.
What fall does a wet-room floor need?
Use the specification for that floor, tray, or tanking system. Trade pages often cite about 1:80 to 1:35. That is their citation, not Table 6 and not a BS 5385 extract. Check the figure in mm/m on a bar that spans the area. A green lamp at level is the wrong instrument.
What is the steepest ramp in Approved Document M?
Table 1 sets maximum gradients: 1:12 for a going of 2 m, 1:15 for 5 m, and 1:20 for 10 m. Steeper than the matching row is over the maximum, not a better fall. A digital level can check the gradient. It cannot sign off width, landings, or handrails. Do not copy a drain gradient onto an access ramp.
Is a pocket inclinometer long enough?
No, not for a pipe or a floor over metres. A pocket box can check a short collar. It cannot stand in for a 6 m run. The Grettz is 1,200 mm, so one placement covers 1.2 m and you still walk the pipe. If the drain is notifiable, stop. The reading does not replace Building Control.

Related pages

Digital spirit level or bubble

Digital spirit level

Best digital spirit level: top pick and best value

Digital spirit level buying guide: body and accuracy

How to use a digital spirit level and flip-check zero

Sources

Public UK sources on product safety, measuring instruments, Class 2 lasers, and faulty goods. Prices, stars, and review counts are the amazon.co.uk listing figures in this set. Prices move. There was no laboratory test. Retrieved 17 October 2026.

  1. Measuring Instruments Regulations 2016, GOV.UK (retrieved 17 October 2026)
  2. General Product Safety Regulations 2005, GOV.UK (retrieved 17 October 2026)
  3. Product safety advice for businesses, GOV.UK (retrieved 17 October 2026)
  4. Laser radiation safety advice, GOV.UK (retrieved 17 October 2026)
  5. HSE, lasers (retrieved 17 October 2026)
  6. Stabila TECH 196 DL (retrieved 17 October 2026)
  7. Trend DLB, manufacturer page (retrieved 17 October 2026)
  8. Consumer Rights Act 2015 (retrieved 17 October 2026)