Septic Tank BOQ — Worked Example for a 10-User IS 2470 Household Tank
A septic tank is the on-site sewage treatment chamber that every Indian household or community without sewer connection uses. It receives black-water from toilets, holds it in two compartments for settling and anaerobic breakdown, and discharges treated effluent to a soak pit downstream. IS 2470 Part 1 specifies the rectangular brick-masonry tank with PCC bed, internal cement plaster + waterproofing, RCC cover slab with manhole, and inlet/outlet RCC pipes. This article walks through a complete BOQ for the most common SBM-G household size — 10 users, 2.0 × 0.9 × 1.3 m.
Project Scenario
You're estimating a septic tank for a single rural household of 10 family members (joint family — grandparents + parents + 3 children + extended). IS 2470 Part 1 specifies 2.0 m × 0.9 m × 1.0 m liquid depth + 0.30 m freeboard for this size. The tank is in 230 mm full-brick masonry on a 100 mm PCC bed, internally plastered with waterproofing, RCC cover slab with two manholes.
Design specification at a glance
- Users: 10 (household-scale per IS 2470 Part 1)
- Internal dimensions: 2.0 m × 0.9 m
- Liquid depth: 1.0 m
- Free board: 0.30 m
- Total internal height: 1.30 m
- Wall thickness: 230 mm (full-brick F.P.S. class-7.5)
- Foundation depth: 0.45 m below NGL
- Cover slab: 125 mm RCC M20
- Waterproofing: internal cement-based crystalline (IS 3370 / IS 2645)
The Complete BOQ — 13 Items in CPWD DSR 2023
| # | DSR Code | Item Description | Unit | Quantity |
|---|---|---|---|---|
| 1 | 2.8.1 | Earthwork in foundation pit, all-kinds-of-soil | m³ | 8.93 |
| 2 | 4.1.10 | PCC 1:4:8 bed below tank, 100 mm thick | m³ | 0.39 |
| 3 | 6.1.2 | F.P.S. brick masonry in cement mortar 1:6, in foundation | m³ | 1.16 |
| 4 | 4.10 | 40 mm DPC in cement concrete 1:2:4 with bitumen | m² | 1.55 |
| 5 | 6.4.2 | F.P.S. brick masonry in cement mortar 1:6, above plinth | m³ | 2.01 |
| 6 | 5.3 | RCC M20 in cover slab, 125 mm thick | m³ | 0.42 |
| 7 | 5.9.3 | Centering & shuttering for cover slab | m² | 3.35 |
| 8 | 5.22.6 | TMT Fe-500D reinforcement (cut, bent, placed) | kg | 41.80 |
| 9 | 13.1.2 | 12 mm internal cement plaster 1:6 on smooth face | m² | 9.34 |
| 10 | 22.5 | Internal cement-based waterproofing per IS 3370 | m² | 9.34 |
| 11 | 7132 | Cast iron manhole cover with frame, 500 × 500 mm | nos | 2 |
| 12 | 19.6.2 | 150 mm RCC pipe for inlet + outlet runs | m | 4 |
| 13 | 6.12.2 | Baffle wall (small brick partition between chambers) | m³ | 0.78 |
The live BOQ Builder evaluates these formulas in real time — change user count (5 / 10 / 15 / 20 / 30 / 50), liquid depth, wall thickness, or override the internal dimensions, and the quantities update instantly.
How Each Quantity Was Computed
1. Earthwork — Item 2.8.1
Trench has 150 mm working space around external tank dimensions:
External length = 2.0 + 2 × 0.23 = 2.46 m
External width = 0.9 + 2 × 0.23 = 1.36 m
Trench length = 2.46 + 0.30 = 2.76 m
Trench width = 1.36 + 0.30 = 1.66 m
Trench depth = found_depth + PCC + internal_h + cover_slab
= 0.45 + 0.10 + 1.30 + 0.10 = 1.95 m
Excavation volume = 2.76 × 1.66 × 1.95 = 8.93 m³
2. PCC bed — Item 4.1.10
PCC length = external_l + 0.15 = 2.61 m
PCC width = external_w + 0.15 = 1.51 m
PCC volume = 2.61 × 1.51 × 0.10 = 0.39 m³
3-5. Brickwork + DPC
Wall periphery (excluding corner double-counting):
Wall periphery = 2 × (2.46 + 1.36) − 4 × 0.23 = 6.72 m
Foundation brick height = found_depth + plinth band = 0.45 + 0.30 = 0.75 m
Foundation brickwork = 6.72 × 0.23 × 0.75 = 1.16 m³
Above-plinth brickwork = 6.72 × 0.23 × 1.30 = 2.01 m³
DPC area = wall periphery × wall thickness = 6.72 × 0.23 = 1.55 m²
6-8. RCC cover slab + reinforcement
Slab volume = external_l × external_w × 0.125 = 2.46 × 1.36 × 0.125 = 0.42 m³
Slab area (shutter under-side) = 2.46 × 1.36 = 3.35 m²
Steel = slab_volume × 100 = 0.42 × 100 = 41.8 kg
Slab is 125 mm RCC with 8 mm bars @ 150 mm c/c bothways — adequate for the 2.46 × 1.36 m span. For larger tanks (20-user and up), bump slab to 150 mm and steel to 110 kg/m³.
9-10. Internal plaster + waterproofing
All 4 walls + tank base (where the liquid sits):
Wall plaster area = 2 × (internal_l + internal_w) × internal_h
= 2 × (2.0 + 0.9) × 1.3 = 7.54 m²
Base plaster area = internal_l × internal_w = 2.0 × 0.9 = 1.80 m²
Total internal area = 9.34 m²
11-13. Manholes + pipes + baffle
Two manhole covers (one over each compartment for desilting access), 4 m of 150 mm RCC pipe (covering inlet + outlet + branch lengths), small baffle wall between settling and outlet chambers.
IS 2470 Standard Sizes (Quick Reference)
| Users | Length × Width (m) | Liquid depth (m) | Total volume (kL) |
|---|---|---|---|
| 5 | 1.5 × 0.75 | 1.0 | 1.12 |
| 10 | 2.0 × 0.9 | 1.0 | 1.80 |
| 15 | 2.0 × 1.0 | 1.3 | 2.60 |
| 20 | 2.0 × 1.2 | 1.4 | 3.36 |
| 30 | 3.0 × 1.2 | 1.6 | 5.76 |
| 50 | 4.0 × 1.5 | 1.7 | 10.20 |
What This BOQ Excludes
- Vent pipe — 4 m × 50 mm GI pipe (~₹1,500) for septic gas escape, top of one chamber
- Inlet inspection chamber upstream — small brick pit (~₹3,000) where toilet pipe enters septic system
- Downstream soak pit — use the dedicated Soak Pit Builder
- Three-compartment design — for ≥ 20 users, IS 2470 recommends a 3-chamber design with primary settling + secondary clarification + scum hold. Add ~30 % to brickwork and 2 nos extra manholes.
- Tile finish on internal surface — optional for ease-of-cleaning, +₹400/m². Recommended for community / commercial tanks with frequent desilting.
- Toilet drainage line from the house — separate plumbing scope, typically PVC SWR pipe
Common Estimation Mistakes
- Single-chamber tank — IS 2470 mandates 2 compartments minimum (settling + clarification). Single-chamber tanks let solids carry through to the soak pit, clogging it.
- Skipping internal waterproofing — septic tanks in black-cotton or expansive clay soil exfiltrate untreated sewage into surrounding groundwater. IS 2470 + IS 3370 require waterproofing for any tank in expansive or fissured soil.
- Using single manhole — one cover means you can only clean one chamber. IS 2470 requires one manhole over each chamber for proper desilting.
- Under-sizing for "10 users" when 15+ live in house — Indian joint families routinely have 12-16 residents in homes that get a "10-user" septic tank. Tank fills in 2 years instead of 5. Always size for actual occupancy + visitor margin.
- Forgetting plinth band brickwork — the 300 mm plinth band between foundation and above-plinth brickwork is structural (DPC sits on it). Skipping it puts DPC below grade where it's prone to moisture damage.
What Changes for a Different Tank
- 5-user (smallest household) — 1.5 × 0.75 × 1.0 m liquid. Wall thickness can be reduced to 115 mm (half-brick) for cost saving.
- 20-user (joint household / hostel) — 2.0 × 1.2 × 1.4 m. Bump cover slab to 150 mm + 110 kg/m³ steel for the longer span.
- 50-user (community / school) — 4.0 × 1.5 × 1.7 m. 3-compartment design recommended. RCC cover slab needs intermediate beam supports.
- Rocky soil site — switch excavation item 2.8.1 to 2.9.1 (ordinary rock) or 2.9.2 (hard rock with blasting). PCC bed thickness may need to increase for uneven rock surface.
- High water table — adds dewatering (~15-25 % cost increase) + permanent buoyancy check (anchor tank or add weight via thicker slab + base).
- State-PWD SOR — switch from CPWD DSR to your state's PHED / Rural Sanitation SOR — usually more current for SBM-G work.
Get Your Septic Tank Cost in 30 Seconds
Change user count (5-50), internal dimensions, liquid depth, wall thickness, waterproofing toggle — the Builder regenerates the BOQ + downloads Excel with formulas embedded. Customize for Your Project →
References & Companion Reading
- IS 2470 Part 1:1985 — Code of Practice for Installation of Septic Tanks (sizing, materials, construction)
- IS 2470 Part 2:1985 — Drainage and effluent disposal
- IS 456:2000 — Plain and Reinforced Concrete (cover slab design)
- IS 2645:2003 — Integral cement waterproofing compounds
- IS 1200 Part 3:1976 — Brickwork measurement
- IS 1200 Part 12:1976 — Plastering measurement
- Sister tool: Soak Pit BOQ Builder — for downstream effluent disposal
- Sister tool: Toilet Block BOQ Builder — for SBM community toilets
- CPWD DSR 2023 — full SOR with rates by item