Enter all three tanks, not just the fresh one. That is the whole difference between this and every other water calculator, and it changes the answer more often than not: on a typical 44/30/30 gallon rig with two people, the grey tank fills about eight hours before the fresh tank runs dry. Fresh water was never the constraint.
The tank most people plan around is the black one, and on the same numbers it would last fifteen days.
Step 1 of 3
Your tanks
All three, in US gallons — they are on the spec sheet for your model, and usually on a sticker inside a cupboard door. The third one is the reason this calculator exists.
Step 2 of 3
How you use water
Pick the closest starting point, then change any row to what you actually do. Figures are per person, per day.
| Activity | Gallons each, per day | Ends up in |
|---|---|---|
| A "navy shower" — wet, tap off, soap, rinse — is the single biggest lever on this page | Grey | |
| Running the tap costs several times what a basin does | Grey | |
| Grey | ||
| The only thing filling your black tank — which is why it is rarely the one that stops you | Black | |
| Leaves the fresh tank and never reaches grey, so it is counted separately | Used up |
Step 3 of 3 — the answer
You can stay
—
What that water comes through
Camco TastePURE RV & Marine Inline Water Filter with Flexible Hose Protector
Inline · 20 micron · no flow rate published
Check price at Camco(opens in a new tab)CLEAR2O RV & Marine Inline Water Filter
Inline · 1 micron · no flow rate published
Check price at CLEAR2O(opens in a new tab)Beech Lane External RV Dual Water Filter System
Canister · 0.5 micron · no flow rate published
Check price at Beech Lane(opens in a new tab)Clearsource Premier RV Water Filter System
Canister · 0.2 micron absolute · 6 GPM published
Check price at Clearsource(opens in a new tab)Arithmetic on your own figures. The per-activity gallons are stated conventions rather than measurements — nobody publishes how much water a shower uses — so treat them as a starting point and edit them to what you actually do. Tanks are assumed empty of waste and full of fresh at the start, and no tank is ever quite as usable as its rated capacity: the pickup sits above the very bottom of the fresh tank, and a grey tank reads full before it is.
What the calculator is doing
Four steps, all of it arithmetic you can check:
- Gallons per person per day, added up from the activity rows. Pick the closest starting point and then correct any row to what you actually do.
- Where each activity’s water goes. Showers and sinks fill grey; flushing fills black; drinking and cooking water is consumed and reaches neither.
- Days for each tank — fresh capacity divided by total use, grey capacity divided by grey input, black by black input.
- The shortest of the three wins, and that one is your trip.
Step 2 is where the useful answer comes from. Total daily use tells you how fast the fresh tank empties and nothing at all about which tank stops you.
Grey is what ends the trip
Every combination tested for this page came back the same way. Three usage levels, three sizes of rig:
| Rig and usage | Fresh lasts | Grey lasts | Black lasts | You can stay |
|---|---|---|---|---|
| 44/30/30, 2 people, conserving | 3 days | 2½ days | 15 days | 2½ days |
| 44/30/30, 2 people, normal | 1½ days | 1 day | 10 days | 1 day |
| 44/30/30, 2 people, no restraint | 21 hours | 16 hours | 7½ days | 16 hours |
| 100/65/50, 2 people, normal | 3½ days | 2½ days | 16½ days | 2½ days |
| 20/15/10 van, 1 person, conserving | 2½ days | 2½ days | 10 days | 2½ days |
Grey bound in all of them. The reason is structural rather than coincidental: almost everything you use water for drains into grey, and grey tanks are usually smaller than fresh tanks. A bigger fresh tank does not help unless the grey tank grows with it.
The practical consequence is that the advice to carry extra water is often solving the wrong problem. Jerry cans extend a trip that is limited by fresh water. If grey is your limit, another twenty gallons of fresh water just fills the grey tank sooner.
The black tank is not your problem
On the default figures a 30 gallon black tank lasts fifteen days. On the largest rig in the table it lasts over three weeks. It is the tank people budget around, ration flushes for, and plan travel days by — and it is almost never the one that runs out.
The arithmetic is unremarkable once written down: only flush water enters it, which is a gallon or two per person per day, against a tank usually about the same size as the grey one taking six or more. Black gets the attention because emptying it is the unpleasant job, not because it is scarce.
That matters practically, because rationing flushes to protect the black tank is a real cost — under-flushing is how you get a solid pile under the toilet that no amount of dumping clears. If black is not your binding tank, and the calculator will show you that it is not, then flushing generously is free.
The gallon nobody counts
Water you drink or cook into food leaves the fresh tank and arrives nowhere. It is not grey water; it left with dinner.
This calculator counts it separately, which as far as we can tell nothing else does. It is only about a gallon a day for two people, so it is not dramatic — but it means the grey tank fills about a tenth slower than a naive model predicts, and on a two-day trip a tenth is a few hours. Given that grey is the binding tank in almost every case, a few hours is the difference between leaving after breakfast and leaving after lunch.
The free fix is showers, and it is not close
Showering is both the largest single use of water and one that drains straight into the tank that fills first. That makes it the only lever on this page worth much.
Take the middle row of the table — 44/30/30, two people, “normal” usage at 8 gallons a shower. Trip length: one day. Drop showers to 3 gallons, which is a navy shower — wet down, tap off, soap, rinse — and:
| Before | After | |
|---|---|---|
| Grey lasts | 1 day | 2 days |
| Fresh lasts | 1½ days | 2 days |
| You can stay | 1 day | 2 days |
It doubles the trip, for no money and no equipment. Nothing else in the list comes near: washing up in a basin rather than under a running tap is the next largest and saves perhaps half a gallon per person per day.
This is why the calculator prints the free fix above the products. A shower head with a shut-off button helps you do it, but the behaviour is the thing that works, and the behaviour costs nothing.
The same shower decides how long the hot water lasts, and by more than most people expect: the hot water calculator shows a 6 gallon heater giving about 4.5 minutes through a 2.5 gal/min head and about 6.6 through a 1.8 — fewer gallons into the grey tank and more minutes from the same tank, from one change.
What that water comes through
Every gallon that goes into your fresh tank arrives through a hose from a spigot you did not choose, at a campground whose water system you know nothing about. That is the honest connection between this page and a purchase — the calculator tells you how many gallons, and a filter is what they pass through.
One thing is worth knowing before you buy, and it is a documentation gap rather than an opinion:
| Filter | Published flow rate |
|---|---|
| Clearsource Premier | 6 to 6.5 GPM |
| Beech Lane dual | Not specified by the manufacturer |
| Camco TastePURE 40043 | Not specified by Camco for this model |
| Clear2O CRV2006 | Not specified by the manufacturer |
One of four tells you how fast water goes through it. For the Clearsource that means a 44 gallon fill takes about seven minutes and a 100 gallon fill about seventeen. For the other three it is not a question the documentation can answer — which is worth knowing if you have ever stood at a spigot wondering why a fill was taking so long.
Whether campground water needs filtering at all is a separate question, and it depends on the type of water system the park runs: is campground water safe to drink splits it the way the regulations do.
Five ways the estimate goes wrong
- Planning on the fresh tank. It is the number everyone quotes and it is usually not the binding one.
- Counting the water heater as capacity. Its six or ten gallons come from the fresh tank and stay in the heater. It is not extra range.
- Assuming rated capacity is usable capacity. The fresh pickup sits above the very bottom of the tank, and a grey tank reads full before it is.
- Rationing flushes. Unless black is your binding tank, under-flushing causes a problem rather than solving one.
- Using someone else’s gallons-per-day. Run it once on the defaults, then correct it against what your own gauges do on the first trip. Everything here is a convention until you have measured yourself.
One thing the tank figures above do not show is weight. Water is 8.34 lb a gallon, so whatever you carry on the road comes out of the trailer’s cargo capacity — the cargo capacity calculator shows how much, and whether arriving with less would fix a trailer that is over its GVWR.
The filters this points at
Four, and the difference is where the filtration sits rather than how many gallons you worked out above. One seasonal note before the picks: everything you have just sized has to come back out before the first freeze, and the winterizing calculator turns the same plumbing into a gallons-of-antifreeze figure — plus the water heater decision, which is worth more than all the fixtures put together.
The default at the spigot
Camco TastePURE 40043
Taste, odour and sediment at the hose for a few dollars — the minimum, not the recommendation.
Finer, same hose fitting
Clear2O CRV2006
A solid carbon block at 1 micron rather than granular carbon at 20, for very little more.
The only one whose maker publishes a flow rate
Clearsource Premier
6 to 6.5 GPM, so a 44 gallon fill takes about seven minutes — knowable for this and nothing else here.
Two stages without the Clearsource price
Beech Lane Dual
Sediment then carbon block, with a lifetime warranty and no published capacity or flow figures.
Related guides
- Is campground water safe to drink? — whether the water going into that tank needs treating, split by the type of system the park runs.
- Best RV water filters — all five compared on what they are certified to do rather than what they claim.
- What micron rating does an RV water filter need? — the sizing question behind the picks above.
- How often should you replace your RV water filter? — capacity converted into months, per product.
- RV battery calculator — the other half of boondocking arithmetic, and usually the other binding constraint.
- The water section collects everything published so far.
For how products on this site are selected and what we do not claim about them, see the editorial policy.
Questions worth answering
How long will a 40 gallon fresh water tank last?
For two people using water carefully, about three days — but that is usually the wrong number to plan on. On a typical 44/30/30 gallon rig the grey tank fills in about two and a half days, roughly eight hours before the fresh tank runs dry, so the grey tank is what ends the trip. Fresh water capacity only becomes the limit if you can dump grey easily or you are showering somewhere else.
Which RV tank fills up first?
Grey, on almost every realistic set of numbers. Everything from the shower and both sinks goes there, which is most of the water you use, and grey tanks are usually smaller than fresh. Across every combination tested for this page — three usage levels, a van, a mid-size trailer and a large fifth wheel — grey was the binding tank every single time. The black tank lasted between seven and twenty-five days.
How much water does one person use in an RV per day?
Between about 8 and 25 gallons depending entirely on showering. Conserving properly — a navy shower, a basin for washing up — comes to roughly 7.75 gallons per person per day. Showering as you would at home takes it to about 25. For comparison, average US household use is somewhere near 80 to 100 gallons per person per day, which is why the first few days off hookups feel like such a change.
Why does my black tank never fill up?
Because only flush water goes into it, and that is a gallon or two per person per day against a tank usually as large as the grey one. On the default figures here a 30 gallon black tank lasts fifteen days while the grey fills in two and a half. Black gets the attention because emptying it is unpleasant, not because it is the constraint.
Does drinking water fill the grey tank?
Mostly not, and this calculator is unusual in accounting for it. Water you drink or cook into food leaves the fresh tank and never reaches a waste tank at all. It is only about a gallon a day for two people, but counting it as grey overstates the grey fill rate by roughly a tenth — enough to move the answer by a few hours on a short trip.
What is the single best way to extend a boondocking trip?
Shorter showers, by a wide margin, because showering is both the largest single use and the one that goes straight into the tank that fills first. On the "normal" usage figures here, dropping showers from 8 gallons to 3 takes a 44/30/30 rig from one day to two — it doubles the trip, for no money and no equipment. Nothing else on the list comes close.
Can I dump grey water on the ground?
It depends entirely on where you are, and it is worth checking rather than assuming. Some public land permits dispersed grey water disposal and much of it does not; many campgrounds prohibit it outright, and rules differ by state and by land management agency. Where it is permitted, using a basin for washing up and carrying it out is the version that keeps soap out of the tank without putting it on the ground near your site.
How long does it take to fill a fresh water tank through a filter?
That depends on the filter's flow rate, and only one of the four filters on this site publishes one. Clearsource states 6 to 6.5 GPM, which puts a 44 gallon fill at about seven minutes and a 100 gallon fill at about seventeen. Camco, Clear2O and Beech Lane publish no flow rate for these models, so the honest answer for those is that it is not knowable from the documentation.
Should I count my water heater in tank capacity?
No — enter the fresh tank only. A six or ten gallon water heater is filled from the fresh tank and stays full, so it is not extra range. It matters for winterizing and for the first fill, not for how many days you can stay.
Are these gallon figures measured?
No, and the page will not pretend otherwise. No manufacturer publishes how much water a shower or a washing-up session uses, so the per-activity figures here are stated conventions chosen to sit within the range RV owners report — roughly 5 to 8 gallons per person per day when genuinely conserving, 15 to 25 with no restraint. Every row is editable, which is the point: run it once on the defaults, then correct it against what your own tank gauges actually do.