To choose the right size water tank, multiply the number of people by their daily use, then by the number of days you must last between supplies. For a Karachi family of four to six, that works out to an overhead tank of 1,000 to 1,500 litres and an underground tank of roughly 500 to 1,500 gallons, depending on how often line water comes and whether you buy tankers.
The rest of this guide shows the arithmetic, with tables you can read straight off, and explains why a tank that is too big causes as much trouble as one that is too small. We clean tanks of every size, and an oversized tank holding stale water is a problem we see often.
Understanding Water Tank Capacities
Karachi uses two units side by side. Plastic tanks are sold in litres. Underground tanks, tankers and the water utility all talk in gallons, and that means the imperial gallon.
| Unit | Equals |
|---|---|
| 1 gallon (imperial) | 4.55 litres |
| 1,000 litres (1 cubic metre) | 220 gallons |
| 1 cubic foot | 28.3 litres, or 6.23 gallons |
| 1,000-gallon tanker | about 4,550 litres |
| 1 litre of water | weighs 1 kilogram |
How to measure a tank you already have
For a rectangular underground tank, measure the inside length, width and water depth in feet, multiply them, then multiply by 6.23 to get gallons.
Example: a tank 6 ft long, 5 ft wide and filled to 5 ft deep holds 6 × 5 × 5 = 150 cubic feet. 150 × 6.23 = about 935 gallons, or 4,250 litres.
For a round tank, the volume is 3.14 × radius × radius × depth. Plastic tanks have the capacity printed or moulded on the side.
Factors to Consider When Choosing a Water Tank Size
1. How many people, and how much each uses
The WHO says between 50 and 100 litres per person per day are needed to meet basic needs (UN). A city household with flush toilets, showers and a washing machine uses more. A sensible planning figure is 100 to 150 litres per person per day. A typical breakdown looks like this:
| Use | Litres per person per day |
|---|---|
| Drinking and cooking | 10 |
| Bathing and ablution | 45 to 55 |
| Toilet flushing | 30 |
| Washing clothes | 20 |
| Washing dishes | 10 |
| House cleaning and other | 10 |
| Total | about 125 to 135 |
Add extra for a garden, car washing, a servant quarter or frequent guests.
2. How often water arrives
This is the factor that makes Karachi different. The city needs about 1,200 million gallons a day and gets about 650 (Geo News), so supply is rationed. Some areas get line water on alternate days, some twice a week, some once a week or less. Your underground tank has to cover the longest normal gap, with a margin.
3. Whether you buy tankers
Tankers come in fixed loads, commonly 1,000, 2,000 and 3,000 gallons. Your underground tank must be able to take a full load on top of whatever is left in it. A 900-gallon tank cannot accept a 1,000-gallon tanker, and you pay for water that runs into the street. If you rely on tankers, build at least 1,500 gallons.
4. Space and structure
The plot limits the underground tank and the roof limits the overhead one. A full 2,000-litre tank weighs two tonnes and must sit over a beam or column. See how to choose the right location for your tank.
5. Future needs
Allow for a growing family or a second portion, but do not double the size “just in case”. Stored water has a shelf life.
The Sizing Formula
Daily use = people × 135 litres
Overhead tank = one day’s use, rounded up to the next standard size
Underground tank = daily use × days between supplies × 1.2 (the extra 20% is a reserve for a late supply)
Worked examples
| Household | Daily use | Overhead tank | Underground, supply every 3 days | Underground, supply weekly |
|---|---|---|---|---|
| 2 people | 270 L (60 gal) | 500 L | about 1,000 L (220 gal) | about 2,300 L (500 gal) |
| 4 people | 540 L (120 gal) | 1,000 L | about 1,950 L (430 gal) | about 4,500 L (1,000 gal) |
| 6 people | 810 L (180 gal) | 1,000 to 1,500 L | about 2,900 L (640 gal) | about 6,800 L (1,500 gal) |
| 8 people | 1,080 L (240 gal) | 1,500 L | about 3,900 L (860 gal) | about 9,100 L (2,000 gal) |
| 10 people | 1,350 L (300 gal) | 2,000 L | about 4,900 L (1,070 gal) | about 11,300 L (2,500 gal) |
If you buy tankers, compare the underground figure with the tanker load you order and take the larger of the two, plus a few hundred gallons of headroom.
Minimum and standard sizes for a house
- Minimum: a 500-litre overhead tank and a 400 to 500-gallon underground tank will serve two or three people with frequent supply. Anything smaller runs dry on the first late supply day.
- Standard: most 120 to 240 square yard houses do well with a 1,000 to 1,500-litre overhead tank and an underground tank of 1,000 to 1,500 gallons.
- Large houses and two-family homes: 2,000 to 3,000 litres overhead, often as two linked tanks, and 2,000 gallons or more underground.
Typical set-ups by house size
Plot size does not decide water use, people do, but most houses of a given size hold a similar number of people. These are common, workable combinations:
| House | Typical occupants | Overhead tank | Underground tank |
|---|---|---|---|
| 80 sq yd, single unit | 4 to 5 | 750 to 1,000 L | 500 to 800 gal |
| 120 sq yd, single unit | 5 to 7 | 1,000 to 1,500 L | 800 to 1,200 gal |
| 120 sq yd, two portions | 8 to 12 | 2,000 L, or two of 1,000 L | 1,500 to 2,000 gal |
| 240 sq yd, single unit | 6 to 8 | 1,500 to 2,000 L | 1,500 to 2,000 gal |
| 240 sq yd, two portions | 10 to 14 | 3,000 L in two tanks | 2,000 to 3,000 gal |
| 400 to 500 sq yd | 8 to 12, plus staff and garden | 3,000 to 4,000 L in two tanks | 3,000 gal or more |
Check any of these against the formula using your own numbers. A house that gets line water on alternate days needs less than one that waits a week.
Inside dimensions for common underground capacities
If you are building, these water dimensions give round capacities. Add about a foot of clear space above the water level for the float valve and inlet.
| Capacity | Example inside size (length × width × water depth) |
|---|---|
| 500 gallons | 5 ft × 4 ft × 4 ft |
| 1,000 gallons | 8 ft × 5 ft × 4 ft |
| 1,500 gallons | 8 ft × 6 ft × 5 ft |
| 2,000 gallons | 8 ft × 8 ft × 5 ft |
| 3,000 gallons | 10 ft × 8 ft × 6 ft |
Keep the depth to what a person can work in safely and a pump can lift from easily. A very deep, narrow tank is harder to clean and harder on the pump than a shallower, wider one of the same volume.
Measure your real use
The 135-litre figure is an average. To find your own, fill the overhead tank, switch the pump off, and note the level. Use water normally for 24 hours and note it again. The drop, as a fraction of the tank’s height, multiplied by its capacity, is your household’s daily use. Do it on a normal weekday and again on a washing day.
If you have a garden
A lawn in a Karachi summer needs roughly 5 litres per square metre on each watering day. A 50 square metre lawn therefore takes about 250 litres, as much as two extra people. Size for it, or better, water the garden from a separate tank of boring water or harvested rain.
Why Bigger Is Not Always Better
It is tempting to build the largest tank the plot allows. Resist it, for four reasons:
- Stale water. Chlorine fades and bacteria multiply the longer water sits. If your tank holds two weeks of water, you are always drinking water that is up to two weeks old.
- More sludge. A large floor collects more sediment, and a slow turnover never flushes any of it out.
- Higher cleaning cost. Cleaning charges rise with capacity. Our rates by tank capacity are on the charges page.
- Wasted money and load. Extra concrete below ground and extra tonnes on the roof.
The aim is a tank that empties and refills every two to four days in normal use. If your supply gap forces you to store for a week or more, cleaning on time becomes more important. We recommend every 6 months for all tanks and every 4 months for tanker-fed ones. See how often to clean your water tank.
Common Sizing Mistakes
- Sizing the underground tank below the tanker load. The surplus runs into the street and you have paid for it.
- One huge overhead tank for a small family. The water sits for days in the heat.
- Forgetting the second portion. A floor added later doubles the people on the same tanks.
- Ignoring the roof. Capacity chosen first and structure checked afterwards.
- A deep tank with a tiny manhole. Nobody can get in to clean it.
- A pump that does not match. A small pump on a big overhead tank runs for an hour. An oversized one short-cycles. A well-matched pump fills the overhead tank in roughly 15 to 30 minutes. See pump system maintenance.
- Counting on a bigger tank to fix a supply problem. Storage evens out gaps. It does not create water.
Sizing for Businesses and Buildings
Commercial sizing uses the same method with a different daily figure. Commonly used planning figures are:
| Building | Litres per day |
|---|---|
| Office | 45 per person |
| Day school | 45 per student |
| Restaurant | 70 per seat |
| Hotel | 180 per bed |
| Hospital | 340 or more per bed |
| Apartment building | 135 per resident |
These are starting points. A building’s engineer should confirm them and add any fire-fighting reserve the building requires. For apartment blocks, the question of who looks after the shared tanks is covered in who is responsible for tank cleaning in apartment buildings.
Assessing Water Quality and Purity
Size and quality are linked. The longer water is stored, the more it changes.
- Line water arrives with a little chlorine, which fades in storage. Do not plan to hold it longer than necessary.
- Tanker water often carries sand and silt. A larger tank does not dilute this. It just gives the silt a bigger floor to settle on.
- Boring water in Karachi is hard and often brackish, and it leaves scale. See Karachi’s hard water problem.
Pakistan’s drinking water standards require that no E. coli or coliforms be detectable and that total dissolved solids stay below 1,000 mg/L (S.R.O. 1063(I)/2010). No tank size will make poor water meet them. Treatment and cleaning do that. See filtration and cleaning in Karachi homes.
If you keep boring water and sweet water, store them in separate tanks so that you can use the better water for the kitchen.
Popular Water Tank Types
- Underground concrete tanks for receiving and bulk storage. See pros and cons of underground tanks.
- Multi-layer plastic overhead tanks in standard sizes such as 500, 750, 1,000, 1,500 and 2,000 litres. See benefits of overhead tanks.
- Concrete overhead tanks on apartment buildings and older houses.
- Fibreglass panel and stainless steel tanks for commercial roofs and food-grade use.
The full comparison is in types of water storage tanks and choosing the right tank material.
Legal and Regulatory Considerations
Tanks are part of the building plan. In Karachi, plans are approved under the Karachi Building and Town Planning Regulations, so a new or enlarged underground tank, or a heavy tank on the roof, should be shown on your drawings and checked by your architect or engineer. Housing societies and cantonment areas may have their own rules as well. The water connection itself comes from the Karachi Water and Sewerage Corporation.
We know of no rule that fixes a tank size for a private house. The limits that matter in practice are structural safety and your approved plan.
Maintenance and Care
Whatever size you choose:
- Clean and disinfect both tanks every 6 months.
- Keep lids sealed and vents meshed.
- Check the float valve so the tank fills fully and does not overflow.
- Inspect the inside whenever the tank is empty.
See how to maintain your water tank after cleaning and our service procedure.
Eco-Friendly Water Tank Options
The greenest tank is one sized to turn over quickly, so that no water is thrown away for going stale. Beyond that, a separate tank for rainwater can supply the garden and car washing through the monsoon months. See the benefits of rainwater harvesting and rainwater harvesting and tank cleaning for sustainable living.
Frequently Asked Questions
Is a 1,000-litre water tank enough for a house?
As an overhead tank, yes, for a family of up to six: it covers about a day. As the only storage in a Karachi house, no. You also need underground storage to cover the days between supplies.
What is the average water tank size for a family of four?
A 1,000-litre overhead tank and an underground tank of 500 to 1,000 gallons, or 1,500 gallons if you buy 1,000-gallon tankers.
What is the minimum water tank size for a house?
About 500 litres overhead and 400 to 500 gallons underground for a small household with frequent supply.
What size water tank do I need for a family of six?
About 800 litres a day, so a 1,000 to 1,500-litre overhead tank, and an underground tank of 650 gallons if supply comes every three days or 1,500 gallons if it comes weekly or by tanker.
What size overhead tank suits a double-storey house?
Count the people on both floors. Two families of five need about 1,350 litres a day, so 2,000 litres overhead, ideally as two linked 1,000-litre tanks so one can be cleaned while the other is in use.
What size underground tank do I need for a tanker?
At least the tanker load plus what is usually left in the tank. For 1,000-gallon tankers, build 1,500 gallons.
How many litres are in a gallon in Pakistan?
About 4.55. Pakistan uses the imperial gallon for tanks and tankers.
Does a bigger tank cost more to clean?
Yes. Our rates rise with capacity, from Rs 2,500 for a 400 to 600-gallon tank upward. See the charges page.
Need Your Tank Measured or Cleaned?
If you are not sure what your tanks hold, our team can tell you when they come to clean them. Call or WhatsApp 0313-2796778. We work across Karachi, including Bahria Town, DHA and Scheme 33.
About the author: Ahmar Shoaib is the founder of Karachi Tank Cleaning. He holds a BS in Chemical Engineering from Karachi University and has run the company’s cleaning and disinfection work since 2007. This article follows our editorial policy.
Sources: UN Water for Life Decade media brief, citing WHO on water needs per person; PCRWR figures on Karachi’s water demand and supply, reported by Geo News (May 2025); National Standards for Drinking Water Quality, S.R.O. 1063(I)/2010.
