Septic Tank Size Calculator
For 5 users with yearly cleaning, the IS 2470 WC-only reference size is 1.5 × 0.75 × 1 m liquid depth: 1,125 litres, plus 0.30 m freeboard. For 10 users, it is 2 × 0.90 × 1 m: 1,800 litres.
Choose the number of users and cleaning interval below. The figures come from IS 2470 (Part 1):1985, Appendix A; they are starting references for a designer to check against actual wastewater flow and site conditions.
Find a residential septic tank size
Choose users and cleaning interval to look up an IS 2470 reference size. Intermediate counts use the next listed row, without interpolation.
Leave the ratio blank for the published dimensions. A ratio of 3 means 3:1. The range comes from Part 1, clause 3.4.5.2.
- Published reference
- Table 5: 5 users
- Recommended table liquid capacity
- 1,125 L
- Internal length × breadth
- 1.5 × 0.75 m
- Liquid depth
- 1 m
- Freeboard above liquid
- 0.3 m
- Total internal depth
- 1.3 m
Table 5 assumes WC discharge only. Bath, kitchen and laundry discharge need a separate flow-based design.
IS 2470 (Part 1):1985, Appendix A, Table 5 requires exact design calculations when choosing the tank. These are internal clear sizes; walls, base, cover slab and partition thickness need separate detailing.
Septic tank size as per IS 2470
Use Table 5 for the 5, 10, 15 and 20-user WC-only references. The 50-user row below comes from Table 6 for residential colonies; it has a different flow basis.
| Users / table | L × B (m) | Liquid depth (m) | Liquid capacity (L) |
|---|---|---|---|
| 5 / Table 5 | 1.5 × 0.75 | 1 / 1.05 | 1,125 / 1,181.25 |
| 10 / Table 5 | 2 × 0.9 | 1 / 1.4 | 1,800 / 2,520 |
| 15 / Table 5 | 2 × 0.9 | 1.3 / 2 | 2,340 / 3,600 |
| 20 / Table 5 | 2.3 × 1.1 | 1.3 / 1.8 | 3,289 / 4,554 |
| 50 / Table 6 | 5 × 2 | 1 / 1.24 | 10,000 / 12,400 |
Source: IS 2470 (Part 1):1985, clause 3.4.5.2, Appendix A, Table 5, printed p. 17 and Table 6, printed p. 18. Both specify 300 mm freeboard and call for exact calculations when choosing a tank.
Table 5 excludes bath, kitchen and laundry water. For Table 6, the residential-colony flow assumptions in clause 3.4.1.2 / Table 3 include a WC, bath and wash basin/kitchen sink per household. Check what actually discharges to your tank.
Between listed counts, the calculator chooses the next larger row without interpolation. This is our lookup policy. Above 50 users it returns no dimensions; use the standard’s design approach and an engineered tank. The standard itself also covers larger installations.
Septic tank capacity calculation: 5 and 10 users
Multiply internal length × breadth × liquid depth × 1,000 to get litres. Freeboard is the air space above the liquid and is excluded from this capacity.
Family of 5: WC discharge only
With yearly cleaning: 1.5 × 0.75 × 1.00 = 1.125 m³ = 1,125 L. Internal depth including freeboard is 1.00 + 0.30 = 1.30 m.
With cleaning every two years: 1.5 × 0.75 × 1.05 = 1.18125 m³ = 1,181.25 L. Internal depth is 1.05 + 0.30 = 1.35 m.
10 users: WC discharge only
With yearly cleaning: 2.00 × 0.90 × 1.00 = 1.8 m³ = 1,800 L. Internal depth is 1.00 + 0.30 = 1.30 m.
With cleaning every two years: 2.00 × 0.90 × 1.40 = 2.52 m³ = 2,520 L. Internal depth is 1.40 + 0.30 = 1.70 m.
All example dimensions: Part 1, Table 5 and its freeboard note. These are liquid-space calculations; do not use them as excavation or concrete quantities.
An optional length:breadth ratio changes the plan geometry while retaining at least the table footprint and depth. The calculator enforces 2:1–4:1 and 0.75 m minimum breadth from clauses 3.4.5.1–3.4.5.2. The resulting layout needs a hydraulic and structural check.
Soak pit size needs a soil test
A soak pit is sized by the soil’s ability to absorb daily effluent. Septic tank litres describe storage, so they cannot be used as litres per day.
Under IS 2470 (Part 2):1985, clause 5.1 and Table 2, required absorbing area = daily effluent ÷ allowable application rate. For a circular pit, diameter = required area ÷ (π × effective depth). Only the effective sidewall counts; its depth starts 150 mm below inlet invert. Clause 5.2.1 sets a minimum cross-section of 0.90 m and depth of 1 m below inlet invert.
Check circular soak pit geometry
Soak pit size depends on effluent flow and the soil test, not septic tank storage capacity. Use site measurements and your plumbing designer’s daily flow.
IS 2470 (Part 2):1985, clause 5.1, Table 2: for a time between rows we use the next slower row. Confirm site suitability under clauses 3.1–3.3, 4.2 and 5.3.
Enter the design daily effluent flow, measured percolation time, chosen depth of at least 1 m below inlet invert, and the shallowest seasonal groundwater depth. User count alone cannot size a soak pit.
Part 2 Table 2 excludes soakaways where percolation exceeds 30 minutes. Table 1 directs sites with groundwater within 1.8 m of ground level to other disposal arrangements. Passing these two checks does not establish site suitability.
What to check on site before construction
Have the designer check connected flows, levels, access and effluent disposal before fixing excavation dimensions. The reference tank volume alone cannot settle these details.
- Inlet and outlet levels: Part 1, clauses 3.4.5.4–3.4.5.5 distinguish inlet/outlet details by width. Clause 3.4.5.5 gives an outlet invert 50 mm below the inlet invert for its narrow-tank dip-pipe arrangement; wider tanks use the specified weir arrangement.
- Compartments: clause 3.4.6 permits two chambers above 2,000 L. If divided, the first chamber has twice the second’s capacity. Confirm the compartment arrangement in the project design.
- Freeboard and access: clauses 3.4.8–3.4.9 specify minimum 300 mm freeboard and access to each compartment. Keep access available for desludging.
- Ventilation: clause 3.4.10 specifies a vent pipe at least 50 mm in diameter with mosquito-proof mesh. The designer must choose its termination height and position to avoid smell nuisance.
- Drinking-water protection: Part 2, clause 5.3 sets at least 18 m from a drinking-water source such as a well and 6 m from a habitable building for the subsurface absorption system. These are disposal-system setbacks, not a blanket clearance for every septic tank or water tank. Soil conditions may demand more separation.
What belongs in a septic tank BOQ?
A septic tank BOQ measures the construction around the liquid space. Record the items below from the civil and plumbing drawings; capacity alone gives none of these quantities.
| Work item | Unit |
|---|---|
| Excavation and backfill, separately | m³ |
| PCC bed | m³ |
| Brick masonry or RCC walls and base, separately | m³ |
| Internal / external plaster and waterproof finish | m² |
| RCC cover slab | m³ |
| Reinforcement | kg |
| Formwork | m² |
| Inlet, outlet and ventilation pipes | m |
| Access covers and pipe fittings | number |
Measure the soak pit or other disposal system separately. For an itemised measurement sheet, use BOQ from your drawings. Check storage separately with the water tank capacity calculator, and use the house construction cost calculator for the wider project budget.
Septic tank size FAQs
Use the published tank sizes within their stated wastewater assumptions, then check the design and disposal system for your site.
What is the septic tank size for 10 persons?
IS 2470 (Part 1):1985 Table 5 gives internal length 2 m and breadth 0.90 m. Liquid depth is 1 m for yearly cleaning (1,800 L) or 1.40 m for two-year cleaning (2,520 L). Add 0.30 m freeboard. These sizes assume WC discharge only and require an exact design check.
What size septic tank does a family of 5 need?
The Table 5 reference is 1.5 × 0.75 m internally, with liquid depth 1 m for yearly cleaning or 1.05 m for two-year cleaning. That gives 1,125 L or 1,181.25 L respectively, plus 0.30 m freeboard. It applies to WC discharge only.
How is septic tank capacity calculated?
Liquid capacity in litres = internal length × breadth × liquid depth in metres × 1,000. Exclude freeboard from liquid depth. This checks the volume of a chosen size; it does not replace the flow, settling and sludge calculations in IS 2470 Part 1 clause 3.4.
What happens for 7 users or more than 50 users?
Seven users select the 10-user row. Between listed counts the tool selects the next larger row; that is our lookup policy, not a rule prescribed by IS 2470. Counts from 21–49 select the separate Table 6 residential-colony reference for 50 users. Above 50, this tool gives no size: have the tank engineered using the standard’s design approach.
Does every septic tank need two compartments?
Part 1 clause 3.4.6 says a tank exceeding 2,000 L may be divided into two chambers. If divided, the first chamber is twice the capacity of the second. This clause does not say every tank must have two chambers; confirm the arrangement with the designer and local requirements.
Can soak pit size be calculated from the number of users?
Not reliably on its own. Part 2 clause 5.1 and Table 2 use soil percolation and effluent application rates. You need daily effluent flow, tested soil conditions, groundwater levels and a suitable site. The optional check uses circular sidewall area, excluding the bottom; it is not site approval.