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Approved Document H method

Percolation Test Timer & Vp Calculator

Time the prescribed 150 mm water-level drop, store multiple field readings and convert the average into the Vp value used to size a septic drainage field.

Method page reviewed 3 September 2026

Field timer

The timer continues from its stored start time if you close or refresh the page.

150 mm drop
00:00:00
Approved Document H describes timing the water level as it falls from 75% full to 25% full (a 150mm drop), then dividing the time in seconds by 150 to obtain Vp in sec/mm.

Test readings

Carry out at least three tests with at least two trial holes. Use the boxes for the time taken for each 150 mm drop.

Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Calculated percolation value
:
:Average 150 mm drop time
12 to 100Conventional AD H Vp range
Vp = average seconds ÷ 150 mm
Use in system spec

The test method, without the usual ambiguity

Approved Document H says the percolation test hole should be 300 mm square, excavated to a depth 300 mm below the proposed invert level of the effluent distribution pipe. It then describes filling that section to at least 300 mm depth and allowing it to seep away overnight before the timed test.

  1. Carry out the preliminary site assessment and trial-hole groundwater check first.
  2. Prepare the 300 mm square test hole at the proposed drainage-field level and remove loose debris.
  3. Fill the 300 mm section with water to at least 300 mm and leave it to seep away overnight.
  4. Next day refill to at least 300 mm.
  5. Time the fall from 75% full to 25% full: 150 mm.
  6. Divide the time in seconds by 150. That is the reading's Vp in sec/mm.
  7. Repeat at least three times using at least two trial holes and average the readings.
Weather matters. Approved Document H says the test should not be carried out during abnormal conditions such as heavy rain, severe frost or drought.

Why 12 to 100 sec/mm matters

The Approved Document states that conventional drainage-field disposal should only be used when the average Vp is between 12 and 100 and when the preliminary site assessment and trial-hole tests are favourable. Faster infiltration can allow insufficient treatment; slower infiltration can prevent effective disposal.

The trial hole is not optional context

The same guidance calls for a trial hole to establish the standing groundwater table. It says the groundwater table should not rise to within 1m of the invert level of the proposed effluent distribution pipes, with likely winter levels considered if testing is done in summer.

2026 England update: engineered ground

The Environment Agency updated its ground-discharge guidance in July 2026 to add conditions under which engineered ground used to reduce the percolation rate can meet the general binding rules. The official guidance states a minimum engineered-ground Vp of 15 and requires the system to be designed and constructed in line with the other BS 6297 recommendations. This is a specialist design route, not a reason to alter an ordinary site-test result.