Water Capacity Calculator: Gallons, Days and Payload
Plan on about 3 gallons per person per day for drinking, cooking and washing, plus a 20 percent reserve. Two people for five days is 36 gallons, and at 8.34 lb per gallon that is 300 lb of payload, which is more than a 200Ah lithium bank, a 400W solar array and a fridge put together. Water is the heaviest single item in most overland builds.
Water is the heaviest thing most overland builds carry, and almost nobody counts it until the vehicle feels wrong. One US gallon weighs 8.34 lb, so a 30 gallon tank is 250 lb, which is more than a 200Ah lithium bank, a 400W solar array, an inverter and all the cable put together. Sizing a water system is therefore two questions at once: how much do you need, and how much can the vehicle actually carry.
Water capacity calculator
The reserve exists because plans change and taps are not always where the map said they were. Twenty percent is a sensible default and 30 percent is not paranoid in remote country.
How much water does a person actually use in a day?
Between 1 and 15 gallons per person per day, depending entirely on how the vehicle is set up. That range is uselessly wide until you break it down, and the breakdown is where the planning happens.
| How you live in the vehicle | Gallons per person per day | Two people, 5 days | Weight, lb | Notes |
|---|---|---|---|---|
| Minimal, no on-board shower | 1 to 2 | 10 to 20 gal | 83 to 167 | Drinking, cooking, washing up and a wipe down. Achievable and not pleasant every day. |
| Moderate, sink and outdoor rinse | 2.5 to 3.5 | 25 to 35 gal | 209 to 292 | The realistic figure for most van and truck-camper builds. |
| Comfortable, indoor shower every other day | 4 to 6 | 40 to 60 gal | 334 to 500 | A short shower is 2 to 4 gallons on its own. |
| Full hookup habits carried off-grid | 8 to 15 | 80 to 150 gal | 667 to 1251 | What happens when nobody changes their habits. Almost no vehicle can carry this. |
Look at the weight column for the bottom row. Two people carrying full hookup habits off-grid for five days would need 400 to 750 lb of water, which exceeds the entire payload capacity of many vehicles on its own. That is not a criticism of anyone's washing habits, it is the reason off-grid vehicles have small sinks, foot pumps and outdoor showers. Habits are the cheapest water tank there is.
| Use | Gallons | Notes |
|---|---|---|
| Drinking water, per person per day | 0.5 to 1.0 | More in heat and at altitude. Never the item to economise on. |
| Cooking and coffee, per person per day | 0.3 to 0.6 | Includes the water that ends up in the pasta. |
| Washing up, per person per day | 0.5 to 1.0 | A basin rather than running water roughly halves it. |
| Hand and face washing, per person per day | 0.3 to 0.8 | A foot pump uses far less than an electric demand pump. |
| Navy shower, per shower | 2.0 to 3.0 | Wet, stop the water, soap, rinse. |
| Ordinary short shower, per shower | 4.0 to 8.0 | A 2.0 GPM shower head running for three minutes is 6 gallons. |
| Dog, medium, per day | 0.5 to 1.0 | Plus whatever gets spilled from the bowl. |
| Rinsing gear or a dusty dog | 1.0 to 3.0 | Grey water, and it has to go somewhere. |
Two lines in that table do most of the work. A shower is the swing item: a navy shower at 2 to 3 gallons is roughly a day of everything else, and an ordinary short shower at 4 to 8 gallons is two or three days. And washing up is where a basin beats running water by a factor of two for no loss of comfort at all.
What does the water weigh, and can you carry it?
Fresh water weighs 8.34 lb per US gallon. That single figure is the reason this calculator sits on a 12V power site: water competes directly with the battery bank, the solar array and the tent for the same payload budget, and it usually wins on weight without anyone deciding that it should.
| Capacity | Full weight, lb | Equivalent in 100Ah LiFePO4 batteries | Days for two people at 3 gal each |
|---|---|---|---|
| 5 gal, one can | 42 | 1.6 | 0.8 |
| 10 gal | 83 | 3.2 | 1.7 |
| 15 gal | 125 | 4.8 | 2.5 |
| 20 gal | 167 | 6.4 | 3.3 |
| 30 gal | 250 | 9.6 | 5.0 |
| 40 gal | 334 | 12.8 | 6.7 |
| 60 gal | 500 | 19.2 | 10.0 |
| 100 gal | 834 | 32.1 | 16.7 |
The third column is the one worth staring at. Thirty gallons of water weighs the same as almost ten 100Ah lithium batteries, which is roughly 12kWh of storage, and it buys five days for two people. Nobody would fit ten batteries without thinking hard about it, and plenty of people fit a 30 gallon tank without thinking about it at all. Run your own figures through the payload calculator with the water included before you buy a tank.
Placement matters as much as quantity. A tank low and near the centre of the wheelbase barely changes how the vehicle behaves. The same tank high up, or behind the rear axle, changes the centre of gravity and loads the rear axle disproportionately, which can put a vehicle over its rear gross axle weight rating while still under GVWR overall. The payload and GVWR guide covers the axle limits properly.
Fixed tank or portable cans?
The honest answer for most builds is both, and the reason is payload rather than plumbing.
A fixed tank such as a Ameri-Kart 20 Gallon RV Fresh Water Tank feeds a pump and a sink properly, puts the weight where you choose to put it, and is far more pleasant to live with day to day. Its weakness is that it is either full or you are carrying a partially full tank that sloshes, and you cannot easily leave the weight at home for a short trip.
Portable containers such as a Scepter 5 Gallon Water Can with Easy-Pour Spout solve exactly that. Five gallons is 42 lb, so you carry two for a weekend and six for a week, and the vehicle only ever hauls what the trip needs. They are also trivial to fill from any tap, which matters more in practice than it sounds, and they double as the backup when a pump or a fitting fails. Their weakness is that pouring from a can is not a sink.
A common and sensible layout is a modest fixed tank of 10 to 20 gallons feeding the sink and the pump, plus cans carried only for longer trips. That keeps everyday convenience while leaving the payload decision under your control trip by trip.
How is the water system wired?
A water system is a 12V circuit, and it is a good illustration of why average consumption and peak current are unrelated numbers.
A SHURFLO Revolution 4008-101-E65 RV Fresh Water Pump (3.0 GPM, 55 PSI) is the default 12V demand pump in van and RV builds. It draws roughly 7 to 8A while running, at 55 PSI and 3.0 GPM, and it runs for seconds at a time when a tap opens. A whole day of normal use is under 2 amp-hours, which is trivial next to a fridge. But the conductor and the fuse are sized for the 8A peak, which on a 10 ft run means 12 AWG cable and a 15A fuse from a FASTSTORM 12 Way Blade Fuse Block with LED Indicators (12V) , not the thin wire the daily total might suggest. The wire gauge calculator will confirm it for your own run length.
Two additions are worth the amp-hours. An accumulator tank smooths the pressure so the pump cycles less, which is quieter and slightly more efficient. And a proper inline strainer before the pump keeps grit out of the valves, because a pump that will not seal runs continuously and turns a 2 amp-hour a day system into a 20 amp-hour one overnight.
Where a water system genuinely gets expensive in power is heating. An electric water heater or a heated hose is a large continuous load, frequently 300W or more, which is 23 amp-hours per hour of operation. Almost every off-grid build that wants hot water uses a diesel or propane heater for that reason. Run any electric option through the power consumption calculator before you fit it, because the number is usually decisive.
Freezing is the other failure mode. Water expands as it freezes and will split a tank, a pump head or a fitting, and a tank that freezes solid is also several hundred pounds of ice you cannot use. Drain the system before storage in cold weather, insulate or heat any under-floor tank, and keep the pump and the runs inside the heated envelope wherever the layout allows. The winter power guide covers cold weather systems as a whole.
What about grey water and filtration?
Grey water is the sink and shower water on its way out, and it is roughly 70 to 90 percent of what goes in, since drinking and cooking consume some of the total. It weighs the same 8.34 lb per gallon on the way out as it did on the way in, so a build that carries grey water is carrying that weight twice on the same trip. Disposal on the ground is prohibited in a growing number of areas and is poor practice anywhere near a watercourse, so plan a container and a disposal point rather than an improvisation.
On filtration, the practical approach in a vehicle is to separate drinking water from everything else. A small inline filter handles taste and sediment for the whole system, and a dedicated drinking container or a point-of-use filter handles the water you actually drink. That means you can fill the main tank from a wider range of sources without worrying about all of it. Sanitise the tank periodically, because a warm tank of standing water in a vehicle is exactly the environment biology likes.
Everything on this page is researched guidance drawn from published manufacturer specifications, not professional advice and not a substitute for local regulations on water disposal or for the installation instructions supplied with your pump and tank. Confirm your own payload figures on the door jamb sticker, and treat every water figure here as a planning estimate to be checked against how you actually travel.
Where to go next
- Payload calculator, where the water weight lands
- Overland water system guide
- Power consumption calculator, for the pump circuit
- Wire gauge calculator
- Winter power guide, including freeze protection
Frequently asked questions
How much water do two people need for a week off-grid?
At a moderate 3 gallons per person per day with a sink and an outdoor rinse, two people for seven days is 42 gallons, and with a 20 percent reserve that is about 50 gallons. At 8.34 lb per gallon that is 420 lb of payload, which exceeds what many mid-size vehicles can spare. Most builds solve it by carrying less and refilling more often rather than by fitting a larger tank.
How much does water weigh?
One US gallon of fresh water weighs 8.34 lb. A 20 gallon tank is 167 lb full, a 40 gallon tank is 334 lb, and a 5 gallon jerry can is 42 lb. That makes water the heaviest single item in most overland builds, heavier than a 200Ah lithium bank, a 400W solar array and an inverter combined. It is also the only heavy item that gets lighter as the trip goes on.
Is a fixed tank or portable cans better?
Portable cans let you carry water only for the trip you are actually taking, which is a payload strategy rather than a plumbing one, and they are easy to fill from any tap. A fixed tank is more convenient day to day, feeds a pump and a sink properly, and puts the weight low and centred where the vehicle prefers it. Many builds run both: a modest fixed tank plus cans for longer trips.
How much power does a water system use?
Very little in amp-hours and a surprising amount in peak current. A 12V demand pump draws 7 to 8A while it runs but only runs for seconds at a time, so a typical day is well under 2 amp-hours. That average is irrelevant to the wiring: the circuit must be sized and fused for the 8A peak, which means 12 AWG cable and a 15A fuse on a normal run, not the tiny conductor the daily total might suggest.
Where should the tank go in the vehicle?
As low and as close to the centre of the wheelbase as the layout allows. A 30 gallon tank is 250 lb, and 250 lb high up or behind the rear axle changes how the vehicle handles far more than the same weight low and central. Under-floor tanks are excellent for weight distribution and vulnerable to freezing and to impact, so they need protection and, in cold climates, insulation or heating.
Do I need to carry grey water?
In many places yes, and the requirement is getting stricter rather than looser. Grey water is the sink and shower water on its way out, and disposing of it on the ground is prohibited in a growing number of areas and is bad practice everywhere near a watercourse. Plan roughly 70 to 90 percent of your fresh usage as grey, and remember that grey water weighs the same 8.34 lb per gallon on the way out.
How we choose: we compare published manufacturer specifications, standards documents including ABYC E-11, and verified owner reviews. We do not test gear in person. Vehicle payload, roof load and charging limits vary by model and year, so confirm yours against the door jamb sticker and the owner's manual rather than any number published here.
Totalling your own build weight against your payload? The Overland Build & Power Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.