Grey is not the leftovers of fresh
The intuition everybody starts with is that the grey tank collects what the fresh tank gave out, so an equal pair of tanks lasts the same length of time. It does not, for two reasons that both push the same way. Some of the fresh water never comes back — it leaves in a cup, or inside the food, or as steam off a griddle — which would make grey the slower of the two. And a great deal of what goes down the drain never came out of the fresh tank at all. Ice arrives frozen and leaves as water. Drinks come back half full. A bag of thawed product carries water into the sink. Condensate off a cold well runs to the same place.
At the figures in the form the second effect wins comfortably. Fresh leaves at 0.13 gallons a cover and grey arrives at 0.155, a ratio of 1.19 to 1, so a 42 gallon grey tank standing next to a 40 gallon fresh tank still fills first — at 4.69 hours against 6.32 for the fresh side. Service stops after 210 covers with 9.6 gallons of drinking water still aboard, roughly 73 covers of water you carried both ways.
The hourly draw is not per cover
Hand sinks and sanitiser buckets consume by the hour whether or not anybody is buying, and the form keeps that term separate for a reason. At 45 covers an hour the 1.8 gallons an hour of hourly draw works out to 0.040 a cover, a third of the fresh side. Drop to 15 covers an hour on a wet Tuesday and the same 1.8 gallons is 0.120 a cover, three times as much. A slow day uses more water per order than a busy one, which is the opposite of what most operators expect and the reason a tank figure worked out at a festival does not transfer to a street pitch.
The truck gets heavier as it serves
Because grey gains more than fresh loses, the water aboard grows across the day. The default truck starts with 342 pounds of water and is carrying 395 by the time the grey tank calls a halt — 53 pounds up, arriving late in the day when the propane bottles are lightest and the product is gone. It is not a large number next to a truck, but it moves in the direction nobody expects, and the axle it sits over is usually the rear one.
What the levers actually do
When grey is the binder, adding fresh capacity buys exactly nothing. Ten more gallons of grey buys 60 covers at these rates; ten more gallons of fresh buys none at all, because the day already ends with fresh in the tank. Halving the ice melt and the dumped drinks is worth 27 covers and costs nothing but a drain pan and a rule about where half-finished cups go, and taking a fifth off the water used per order is worth 26. The order of those three is the whole point of running the two clocks side by side rather than sizing off the fresh tank.
Where this stops
It assumes a steady rate of orders, which no lunch rush is, and it assumes nothing goes wrong with a pump. It knows nothing about what tank sizes your operation is required to carry, what ratio between them a health department wants to see, how the truck must be plumbed, or where grey water may legally be emptied. Those are local rules, they differ across a county line, and the department that issued your permit is the only place to get them.
Questions people ask
Why does the grey tank fill before the fresh tank empties?
Because water reaches the grey tank from places other than the fresh tank. Melting ice, drinks handed back, water out of thawed product and condensate all go down the same drain without ever having been in the fresh tank, and on a truck selling cold drinks that inflow is substantial. At the default figures fresh leaves at 0.13 gallons a cover and grey arrives at 0.155, so a slightly larger grey tank still fills first.
How many customers can a food truck serve on one water fill?
At the figures in the form — a 40 gallon fresh tank and a 42 gallon grey tank both usable to 90 percent, 5 gallons of spare in jugs, 4 gallons spent opening and closing, 1.8 gallons an hour of hand sink and sanitiser and 0.09 a cover of order water — it is 210 covers, and the grey tank is what stops it after 4.69 hours. Change the covers an hour and the answer moves, because the hourly draw is spread over more or fewer orders.
Does a slow day use less water per customer?
No, more. Hand sinks and sanitiser buckets run on the clock rather than on the order count, so the hourly draw is divided across fewer covers. At 45 covers an hour the 1.8 gallons an hour works out at 0.040 a cover; at 15 covers an hour the same water is 0.120 a cover. Tank figures measured on a busy festival day flatter a slow street pitch badly.
How much does the water on a food truck weigh?
A gallon of water is 8.345 pounds, so a 40 gallon fresh tank filled to 90 percent is 300 pounds before you add jugs or count anything in the grey tank. The page shows the figure at open and again when service stops, and it goes up rather than down across the day. What your truck is rated to carry is on its manufacturer plate and on the registration, and the axle rating matters as much as the total — this page states no limit and issues no verdict on load.
What size grey tank does a food truck need?
That is not a question a calculator can answer, because it is set by the health department that permits the unit and the requirement is not the same from one jurisdiction to the next. What this page can tell you is which tank ends your day at your own numbers, and how many covers a change in either capacity would buy. Take the operational answer here and the requirement from the department that issued your permit.