Free calculator
Septic tank size calculator
Work out the septic tank a house needs from the people using it, what drains into it and how often it is emptied: the capacity in litres and gallons, the room it keeps for sludge and scum, its size in feet with the free board, and the split into chambers.
- Free, no sign-up
- Answers as you type
- Works on your phone
Calculator
Size a septic tank
Give the people who use it, what drains into it and how often it will be emptied. The answer is the capacity, the size to build and the chambers.
The tank to build
- Inside size
- 8'-0" × 4'-0"4'-0" of liquid and 1'-0" of free board, 5'-0" deep; holds 3,625 litres
- Chambers
- 5'-4" and 2'-8"The first twice as long as the second
- Flow a day
- 1,200 litresOut to the sewer or a soak pit
- Empty it
- Every 2 yearsOr sooner, once sludge and scum fill half its depth
How the capacity is made up
- Settling10 people × 120 litres a day, held 48 hours
- 2,400 litres
- Sludge and scum10 × 40 litres a year × 2 years × 1.15 for digestion
- 920 litres
- Liquid capacity
- 3,320 litres
No sewer? The 1,200 litres a day goes to a soak pit. In fine sand that needs about 36 sq m of pit wall: two pits 5 ft across and 13 ft deep below the inlet. Clay needs three times that or more, so a percolation test settles it. Keep the bottom 2 m above the groundwater and the pit ideally 30 m from any well or bore.
A guide for sizing and pricing, not a design. Local rules can ask for a bigger tank, as Punjab's EPA does in housing societies, and a qualified engineer settles the final size, the soak pit and where both go.
Reference
Septic tank size for 5, 10, 15 and 20 people
Worked out here with bath and kitchen water going in, 48 hours of detention and emptying every 2 years, beside the tanks IS 2470 recommends for toilet waste alone.
| Users | Flow a day, litres | Sludge and scum, litres | Capacity, litres | Imperial gallons | Inside size |
|---|---|---|---|---|---|
| 5 people | 600 | 460 | 1,660 | 365 | 5'-6" × 2'-9", 5'-0" deep |
| 10 people | 1,200 | 920 | 3,320 | 730 | 8'-0" × 4'-0", 5'-0" deep |
| 15 people | 1,800 | 1,380 | 4,980 | 1,095 | 9'-6" × 4'-9", 5'-0" deep |
| 20 people | 2,400 | 1,840 | 6,640 | 1,461 | 11'-0" × 5'-6", 5'-0" deep |
| Users | Length × breadth | In feet | Liquid, cleaned yearly | Liquid, cleaned every 2 years |
|---|---|---|---|---|
| 5 people | 1.5 × 0.75 m | 4'-11" × 2'-6" | 1 m, 1,125 litres | 1.05 m, 1,181 litres |
| 10 people | 2 × 0.9 m | 6'-7" × 2'-11" | 1 m, 1,800 litres | 1.4 m, 2,520 litres |
| 15 people | 2 × 0.9 m | 6'-7" × 2'-11" | 1.3 m, 2,340 litres | 2 m, 3,600 litres |
| 20 people | 2.3 × 1.1 m | 7'-7" × 3'-7" | 1.3 m, 3,289 litres | 1.8 m, 4,554 litres |
Both add 300 mm (a foot) of free board above the liquid. IS 2470 allows for more sludge than WHO does, so its tanks for toilet waste alone run larger than WHO's method gives for the same use.
Pakistani rules
Minimum septic tanks in Pakistani rules
What Punjab's EPA and Lahore's building regulations ask for by plot size. Where a rule asks for more than the calculator, build to the rule.
| Plot | Building | Inside size | Litres |
|---|---|---|---|
| Under 5 marla | House | 5 × 4 × 4 ft | 2,265 |
| 5 marla | House | 6 × 4 × 4 ft | 2,718 |
| 10 marla | House | 9 × 6 × 4 ft | 6,116 |
| Over 10 marla | House | 10 × 6 × 4 ft | 6,796 |
| 1 kanal | Commercial plaza | 10 × 6 × 5 ft | 8,495 |
| 2 to 3 kanal | Commercial plaza | 12 × 6 × 5 ft | 10,194 |
| 3 to 4 kanal | Commercial plaza | 15 × 6 × 5 ft | 12,743 |
| Over 4 kanal | Commercial plaza | 16 × 6 × 5 ft | 13,592 |
| House plot | Inside size | Litres |
|---|---|---|
| Under 1 kanal | 8'-0" × 4'-0" × 4'-3" | 3,851 |
| 1 to 2 kanal | 9'-0" × 4'-6" × 4'-3" | 4,874 |
| Over 2 kanal | 10'-0" × 5'-0" × 4'-3" | 6,017 |
Sizes are as the rules give them, in feet, and the litres are worked from them, so they include any free board. DHA Karachi asks for at least 95 cft and 7 days' retention, 4 ft from the underground water tank. DHA Islamabad-Rawalpindi keeps a tank 3 ft from the front boundary wall and 2 ft from the next house.
How it is worked out
From people to a tank size
The calculator follows WHO's guide to on-site sanitation. A septic tank holds the wastewater long enough for the solids to settle, which is the detention time, and keeps the sludge that sinks and the scum that floats until the tank is emptied. So its liquid capacity is the people times the wastewater a person a day times the detention time, plus the people times the sludge a person a year times the years between emptyings, times a factor for how well the sludge digests.
With bath and kitchen water going in, the wastewater is most of the water used: WHO takes it as 90%, and Islamabad's rules for housing schemes size sewage treatment for 80%. The calculator starts at 120 litres a person a day, 80% of the 150 litres the water tank calculator uses, within WHO's range of 100 to 200. With the toilets alone, WHO's example is four flushes of a 10 litre cistern, 40 litres.
WHO designs for 24 hours of detention and IS 2470 gives 24 to 48. Eawag recommends 48, as a small tank is stirred by every flush and settles poorly, so the calculator starts there; DHA Karachi's rules ask for 7 days. Sludge and scum build up at about 25 litres a person a year from toilets and 40 with bath and kitchen water. Sludge digests slowly in the cold, and not fully in a tank emptied every year, so WHO keeps more room for it: 1.5 times for yearly emptying and 1.15 for every 2 years where winters stay above 10°C, more where they fall below.
IS 2470 wants at least 1 m of liquid, 750 mm of width and 1,000 litres, a length 2 to 4 times the width, and 300 mm of free board over the liquid; WHO prefers 1.2 to 1.5 m of liquid. The calculator rounds each side up to a whole 3 inches and adds a foot of free board. A partition splits the tank so the first chamber is twice the second, with openings about a foot below the liquid, and a vent pipe of at least 50 mm lets the gas out. Punjab's EPA asks housing societies for three chambers, which the calculator makes half, a quarter and a quarter of the length, keeping the first at least half as Eawag advises. Add the thickness of the partition walls to the length.
What leaves the tank still carries germs, so it goes to a public sewer where there is one, as the LDA and DHA rules require, or to a soakage pit filled with brick bats or boulders. The pit's size depends on how fast the soil takes water: WHO's figures run from 50 litres a day through each square metre of pit wall in coarse sand to 10 in compact clay, and a percolation test on the plot settles it. Local rules and a qualified engineer decide the final design.
On site
Terms you will hear
- Sludge
- The solids that settle on the floor of the tank and slowly digest. A suction tanker takes it out when the tank is emptied, leaving about 25 mm behind to restart the digestion.
- Scum
- The layer of grease, soap and light solids that floats on the liquid. The inlet and outlet pipes dip below it, so it stays in the tank.
- Detention time
- How long the wastewater stays in the tank before it flows out, long enough for the solids to settle: usually 24 to 48 hours. Also called retention time.
- Free board
- The air gap between the liquid and the underside of the cover slab, at least 300 mm, for the gas and the scum.
- Baffle wall
- The partition that splits the tank into chambers, with openings below the scum and above the sludge, so only the settled liquid passes on.
- Soakage pit
- A pit filled with brick bats, boulders or gravel that lets the effluent soak into the ground where there is no sewer. Also called a soakaway or soak well.
- Overflow manhole
- The chamber after a septic tank where its outlet joins the sewer. DHA Karachi and DHA Islamabad-Rawalpindi connect the bath and kitchen drains here rather than to the tank.
Questions
Septic tank questions
Still have a question?
Ask our team during Pakistan business hours, in English or Urdu.
+92 333 2466662Chat on WhatsAppWhat size septic tank do I need for my house?
It depends on how many people use it, what drains into it and how often it is emptied. For a family of 6 with bath and kitchen water going in, held 48 hours and emptied every 2 years: 720 litres a day for 2 days is 1,440 litres, plus 552 litres for sludge and scum, 1,992 litres in all. With 4'-0" of liquid that is 6'-0" × 3'-0" inside and 5'-0" deep with the free board.
What is the septic tank size for 10 persons?
With bath and kitchen water going in, 3,320 litres (730 imperial gallons): 2,400 litres to hold the wastewater for 48 hours and 920 litres for 2 years of sludge and scum. That is 8'-0" × 4'-0" inside with 4'-0" of liquid under a foot of free board, 5'-0" deep, in chambers of 5'-4" and 2'-8". With the toilets alone it comes to 1,375 litres. IS 2470's table, for toilets alone, gives 2 × 0.9 m with 1 to 1.4 m of liquid, 1,800 to 2,520 litres.
How do you calculate septic tank capacity?
Capacity is people × wastewater a person a day × detention time in days, plus people × sludge a person a year × years between emptyings × a factor F. WHO takes the sludge as 25 litres a person a year for toilet waste and 40 with bath and kitchen water, and F as 1.15 for emptying every 2 years in mild winters, 1 at 3 years or more. Divide the litres by 28.3 for cubic feet and by the depth of liquid for the floor area, then keep the length 2 to 4 times the width.
What are the standard septic tank dimensions for a house in Pakistan?
There is no one national size. Since September 2025 Punjab's EPA has asked every new housing society for three-chamber tanks of at least 6 × 4 × 4 ft for a 5 marla house, 9 × 6 × 4 ft for 10 marla and 10 × 6 × 4 ft above that. Lahore's building regulations set 8 × 4 ft with 4'-3" of depth for a house under 1 kanal, and DHA Karachi at least 95 cft with 7 days' retention. Build to the larger of your own figure and the rule where you build.
How often should a septic tank be emptied?
IS 2470 asks for a house tank to be emptied at least every 2 years, and Eawag gives 2 to 5 years. Empty it sooner once the sludge and scum together fill half its depth, and leave about 25 mm of sludge behind to restart the digestion. A longer gap needs more room for sludge: for 10 people with bath and kitchen water, emptying every 3 years takes the tank from 3,320 litres to 3,600 litres.
Should bath and kitchen water go into the septic tank?
It depends on the rules where you build. DHA Karachi and DHA Islamabad-Rawalpindi send only the toilets into the tank and the bath and kitchen water to its overflow manhole, while Lahore's building regulations send all of it through the tank. Bath and kitchen water makes the tank bigger, as it takes the flow from about 40 litres a person a day to 120.
How far should a septic tank be from a water tank or well?
DHA Karachi asks for at least 4 ft between a septic tank and an underground water tank, and DHA Islamabad-Rawalpindi keeps the tank 3 ft from the front boundary wall and 2 ft from the next house. A soak pit should be further off: Eawag advises ideally more than 30 m (about 100 ft) from any well or bore, with its bottom at least 2 m above the groundwater. Build both tanks watertight in RCC, as the DHA rules require.
How big should the soak pit be?
That depends on how fast the soil takes water, which a percolation test on the plot settles. WHO's figures run from 50 litres a day through each square metre of pit wall in coarse sand to 10 in compact clay. For 10 people's 1,200 litres a day in fine sand, at 33 litres, that is about 36 sq m of wall: two pits 1.5 m (5 ft) across and 3.9 m (13 ft) deep below the inlet, as Eawag keeps a pit 1.5 to 4 m deep. Where the soil is tight or the groundwater high, connect to a sewer or use soak trenches instead.
Plan, price and build on Smart Construction
The construction ERP for Pakistani contractors, builders and developers: projects, estimates, billing, labour and site records in one place.
Call +92 333 2466662 during Pakistan business hours, or book online.