About this calculator
Size a septic tank for a house from the number of bedrooms and how often it will be desludged, and work out the french drain length from your percolation test, using SANS 10400-P.
How it’s calculated
Tank capacity comes from SANS 10400-P annex C, table C.1 (method 1, for houses), which gives the capacity for a 24-hour retention period by number of bedrooms and years between desludging. A tank for a single house is never smaller than 1.5 m³ (1,500 L).
The daily sewage flow comes from table 1: 750 L for a 2-bedroom house, 900 L for 3, 1,100 L for 4, 1,400 L for 5 and 1,600 L for 6, plus 15% for stormwater and contingencies. The percolation time from the in-situ test (4.25) gives the rate of application from table 3, from 108 L per m² of drain side for fast-draining soil down to 33 L at 30 minutes; slower soil may not take a french drain at all.
Infiltration area = flow ÷ rate. Trench length = (area − 2 × width × depth) ÷ depth, from 4.8.10; the trench bottom is not counted because it clogs with sludge.
Worked example
With these inputs:
- Bedrooms (count habitable outbuilding rooms as bedrooms)
- 3
- Planned desludging every
- 3 years
- Percolation test time (minutes)
- 9
- Add 15% for stormwater and contingencies
- Yes
- French drain trench width
- 600 mm
- French drain trench depth
- 1000 mm
the result is:
- Septic tank capacity
- 1,500 L
- Daily sewage flow
- 1,035 L/day
- Rate of application (table 3)
- 84.8 L per m² of drain side
- Infiltration area needed
- 12.21 m²
- Trench
- 600 mm wide × 1,000 mm deep
Tips from the trade
- Do the percolation test in the area where the drain will go, at the depth of the trench bottom.
- Inspect the tank once a year and desludge it when needed; don’t wash or disinfect it when desludging.
- Keep cooking oil, wipes and sanitary products out of the system; they don’t break down.
Common questions
What size septic tank do I need for a 3-bedroom house?
Under SANS 10400-P table C.1, 1,500 L if it is desludged every 1 to 3 years, rising to 2,800 L for desludging every 8 years. The minimum for a single house is 1.5 m³.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.