STP Capacity Calculator
Find the right Sewage Treatment Plant size (in KLD) for your housing society, commercial building, hotel, hospital, school or industrial campus. Built on NBC 2016 water demand norms and the CPHEEO Manual on Sewerage and Sewage Treatment (2013).
1Choose your project type
This loads typical occupancies as a starting point. You can edit every number below.
2Residential units
Persons per unit follow NBC 2016 (includes domestic help). Leave counts at 0 if there are no flats.
| Unit type | No. of units | Persons / unit | Residents |
|---|---|---|---|
| Total residents | 0 | ||
3Other occupancies
Add offices, shops, hotel beds, hospital beds, students, workers, restaurant seats and more. Litres per day are pre-filled from NBC 2016 and can be edited.
4Design assumptions
Defaults follow CPHEEO practice. Change them only if you have site-specific data.
How this STP calculator works
Calculation method, step by step
- Count the occupants. Residents are estimated from flat types using NBC 2016 (Studio 2, 1 BHK 4, 2 BHK 5, 3 BHK 6, 4 BHK and above 7, EWS 4 persons). Non-residential load uses actual headcount, beds or seats.
- Calculate water demand. Each occupancy is multiplied by its NBC 2016 per capita norm (litres per head, bed or seat per day).
Water demand (KLD) = Σ (occupants × litres per day) ÷ 1000
- Convert water to sewage. CPHEEO assumes 80% of water supplied reaches the sewer, rising to about 90% in well-developed areas. Direct wastewater streams (laundry, kitchen) are added as entered.
Average sewage (KLD) = Water demand × return factor + direct wastewater
- Add infiltration (optional). For long sewer networks or a high water table, groundwater entering the pipes is added, capped at 10% as per CPHEEO.
- Add a design margin. Typically 10% to 20% for future occupancy and load variation.
Design capacity (KLD) = (Average sewage + infiltration) × (1 + margin)
- Round up to a standard plant size. Plants are built in standard capacities (for example 25, 50, 75, 100 KLD).
- Check peak flows. Sewage does not arrive evenly through the day. CPHEEO peak factors (3.0 up to 20,000 population, 2.5 up to 50,000, 2.25 up to 7,50,000, 2.0 above) size the equalization tank, pumps and pipelines. They are not used to multiply the plant's daily KLD rating.
Water demand norms used (NBC 2016)
| Occupancy | Unit | Domestic | Flushing | Total (L/day) |
|---|---|---|---|---|
| Residential (standard / MIG / LIG) | per person | 90 | 45 | 135 |
| Residential (large cities, premium) | per person | 105 to 155 | 45 | 150 to 200 |
| Visitors | per person | 5 | 10 | 15 |
| Offices (incl. canteen) | per person | 25 | 20 | 45 |
| Factories with bathrooms (incl. canteen) | per worker | 30 | 15 | 45 |
| Factories without bathrooms | per worker | 20 | 10 | 30 |
| Hospital, up to 100 beds* | per bed | 230 | 110 | 340 |
| Hospital, above 100 beds* | per bed | 300 | 150 | 450 |
| Hospital OPD | per patient | 10 | 5 | 15 |
| Nurses' homes, medical quarters, hostels | per person | 90 | 45 | 135 |
| Hotels up to 3 star** | per bed | 120 | 60 | 180 |
| Hotels 4 star and above** | per bed | 260 | 60 | 320 |
| Restaurants (incl. kitchen) | per seat | 55 | 15 | 70 |
| Food courts (incl. kitchen) | per seat | 25 | 10 | 35 |
| Clubhouse | per person | 25 | 20 | 45 |
| Schools without boarding | per person | 25 | 20 | 45 |
| Schools / colleges with boarding | per person | 90 | 45 | 135 |
| Malls: staff / visitors | per person | 25 / 5 | 20 / 10 | 45 / 15 |
| Cinemas, theatres, multiplexes | per seat | 5 | 10 | 15 |
| Stadiums | per seat | 4 | 6 | 10 |
| Airports | per person | 40 | 30 | 70 |
| Interstate bus terminals | per person | 25 | 20 | 45 |
| Metro / intrastate bus stations | per person | 10 | 5 | 15 |
* Includes patients, attendants, visitors and staff. Excludes laundry and kitchen, which should be added separately.
** Excludes laundry, kitchen, staff and water bodies, which should be added separately.
Source: National Building Code of India 2016, Part 9, Section 1, as published by the Central Ground Water Authority.
Frequently asked questions
How is STP capacity calculated?
STP capacity is calculated from the daily water demand of all occupants. Water demand is converted to sewage using a return factor (80% as per CPHEEO), an optional infiltration allowance is added, and a design margin of 10% to 20% is applied. The result is rounded up to a standard plant size and expressed in KLD (kilolitres per day, where 1 KLD = 1,000 litres per day).
What water demand per person is used for STP design in India?
NBC 2016 recommends 135 litres per person per day for standard housing, and 150 to 200 litres for communities above 1 lakh population with full flushing systems. About 45 litres of this is for flushing. Offices and day schools use 45 litres per person per day, and visitors 15 litres.
How many persons per flat should be considered?
NBC 2016 suggests 2 persons for a studio, 4 for a 1 BHK, 5 for a 2 BHK, 6 for a 3 BHK and 7 for a 4 BHK or larger unit, including domestic help. Some local authorities follow a flat figure such as 5 persons per unit, so check the norm used in your approval.
Should the STP capacity be multiplied by the peak factor?
No. The daily KLD rating is based on average daily sewage flow plus a design margin. The peak factor (2.0 to 3.0 as per CPHEEO) is used to size the equalization tank, pumps, pipelines and other hydraulic parts that must handle morning and evening surges.
Does reusing treated water for flushing reduce the STP size?
No. Treated water used for flushing returns to the STP as sewage, so the plant must still treat the full flow. Reuse reduces your freshwater demand and water bill, not the STP capacity.
Can industrial effluent be treated in the same STP?
Generally no. An STP is designed for domestic sewage. Industrial process wastewater has different characteristics and usually needs a separate Effluent Treatment Plant (ETP). This calculator counts only the domestic sewage from workers and staff.
This calculator gives a preliminary estimate for planning and budgeting. Final STP design should be based on the approved project water balance, local authority and Pollution Control Board conditions, actual wastewater characteristics and site constraints. Talk to Brivith Engineering for a detailed design and proposal.