What is a water submeter? How its readings split a bill
Quick answer
A water submeter is a meter placed after the main water meter that measures how much water one unit or tenant uses. A HUD study describes submetering as additional meters downstream of a master meter. To split a bill with the readings, split the fixed charges equally and divide the usage charge in proportion to each reading.
Key points
- A submeter measures one unit's use downstream of the main meter. A HUD study says submeters can measure the water use of an individual apartment.
- Each unit's share is the usage charge times its reading divided by the sum of all readings, plus an equal share of the fixed charges.
- On a $138.00 bill with $18.00 fixed and readings of 4,000, 3,000 and 1,000 gallons, the shares are $66.00, $51.00 and $21.00.
- Readings can add up to less than the main meter, because a shared tap or a leak passes the main meter without passing any unit's submeter.
On this page
What is a water submeter?
A water submeter is a meter that measures the water used by one unit or tenant, installed after the main meter. The U.S. Department of Housing and Urban Development’s study of submetering defines submetering as “the installation of additional metering devices downstream of a master meter that can measure the amount of a utility consumed by a specific entity”, and says: “Submeters can measure the energy (or water) use of an individual apartment.” For multifamily housing, it says, the master meter typically measures a campus of buildings or a single building.
How do you split a bill from submeter readings?
Split the fixed charges equally, then split the usage charge in proportion to each reading. For one unit, the share is the usage charge times the unit’s reading divided by the sum of all readings, plus its equal part of the fixed charges.
Say a three-unit house gets a $138.00 water bill. It lists $18.00 of fixed charges, so $120.00 is the usage charge. The submeters read 4,000, 3,000 and 1,000 gallons for the month, which is 8,000 in all. The fixed charges are $6.00 for each unit.
| Unit | Gallons | Usage share | Pays, with $6.00 fixed |
|---|---|---|---|
| A | 4,000 | $60.00 | $66.00 |
| B | 3,000 | $45.00 | $51.00 |
| C | 1,000 | $15.00 | $21.00 |
Unit A’s usage share is $120.00 times 4,000 divided by 8,000, which is $60.00. The three totals add up to the $138.00 bill. Only the proportions matter, so readings of 4, 3 and 1 give the same shares.
In EvenOut, enter two expenses. Add “Water, fixed” for $18.00 with “Split equally”, and add “Water, usage” for $120.00 with “Shares”, typing 4, 3 and 1. Each person’s dollar share shows under their name.
Why do the readings not add up to the main meter?
Anything that passes the main meter without passing a unit’s submeter shows up as a gap. A shared outdoor tap, a laundry room or a leaking pipe before the units are examples.
Say the main meter shows 8,800 gallons against 8,000 on the three submeters. The 800 gallons in the gap are 9.1 percent of the building’s water, and they are in nobody’s reading. The formula above spreads the whole $120.00 usage charge across the units by their readings, so each unit carries a part of the gap. If the group wants to handle the gap another way, agree it before the bill.
A large gap is worth a look. The HUD study says that with submetering of water, leaks “can be identified and addressed long before severe financial or physical damage occurs.” EPA’s WaterSense page gives a leak test for a water meter: read it when no water is in use, read it again two hours later, and a changed reading means there is probably a leak.
What does it mean for roommates inside one unit?
The unit’s submeter reading sets the unit’s bill, and the roommates in that unit split that bill in whatever way they split water. How to split a water bill shows nights at home as one way, and the utility bill split calculator by usage does the arithmetic from any set of readings. Electric and gas bills follow the same method with their own readings, and how to split electric and gas bills covers their fixed and usage lines.
Frequently asked questions
What if a unit's reading is zero?
A unit with 0 gallons has a usage share of $0.00. If it still belongs to the household, it still pays its fixed share, which is $6.00 in the example. If the group wants a vacant unit to pay nothing, agree that before the bill arrives.
Can the same method split an electric or gas submeter?
Yes. The HUD study says submeters can measure the energy or water use of an individual apartment. Take the fixed charges off the bill, split them equally, and split the rest by each unit's reading in the same unit, such as kilowatt-hours for electricity.
Is a meter reading the use for the month?
A reading is a running total of the water that has passed through the meter, so a month's use is this month's reading minus last month's. Say the dial showed 48,200 gallons last month and 52,200 this month. The use is 4,000 gallons. Use the same unit for every reading.
How do you know which meter a bill is based on?
Ask whoever sends the bill which meter it is based on, and ask for the readings behind it. The unit's own reading is the one to use in the formula, while the main meter shows the whole building.
Sources
Checked October 6, 2026.
Keep reading
- Guide How to split a water bill by who is home Split the fixed charge equally and the usage by nights home. On a $96 bill, four roommates pay $33.00, $33.00, $19.50 and $10.50. Guests and leaks covered.
- Calculator Split utilities calculator: fixed charges plus usage Enter the bill, the fixed charges and each person's usage. The fixed part splits equally, the rest by use, in whole cents that add up to the bill.
- Guide How to split electric and gas bills fairly A heavy device costs its owner kWh times the bill's average rate, and the rest splits equally. Gas swings with the seasons, so use an average or the real bill.
- Guide How to split utilities in a duplex with one meter Two households on one utility account: split by household, not by person. A $240 electric bill is $120 each at 50/50, or $180 and $60 by occupants.