Measure the hose, do not look it up
Published garden hose flow rates are close to meaningless because the flow depends on your supply pressure, the length and diameter of the hose, how far the tap is open, and what else in the house is drawing water at the time. A five eighths inch hose on good pressure might give ten or twelve gallons a minute; the same hose on a long run from a well pump with a filter in line might give four. The difference is the difference between a fifty minute fill and a three hour one.
Timing a bucket takes thirty seconds and removes the guesswork entirely. Use a bucket whose volume you know, run the hose exactly as you will run it for the tub, and time it from a standing start with the bucket already under the stream. Gallons divided by minutes is your flow. If you plan to fill through a hose filter, time the bucket through the filter too, because a filter cartridge can halve the rate.
Where the fill factor comes from
Length times width times depth describes a box, and a tub is not one. The seats are the largest single displacement, since on most spas they run right around the perimeter and are a foot or more deep into the water. The footwell is a smaller rectangle set inside the outline. The floor curves rather than meeting the walls at a corner. And the water line sits below the top of the shell, often by four to six inches, which people forget when they measure the cabinet instead of the water.
Taken together those effects usually put the real volume at 70 to 80 percent of the box figure on a seated spa. A plain soaking tub or a stock tank with near vertical walls runs 85 to 95. Inflatable tubs are lower again, because the wall thickness eats into the footprint from all sides and the floor is rounded. If the manufacturer publishes a capacity in gallons, that number beats anything you can derive here, and you can work backwards to see what fill factor it implies for your shell.
Two hoses, and why it is not twice as fast
Splitting the supply into two hoses adds flow, but not proportionally. Every foot of pipe and hose has friction that rises with the square of flow, so doubling the draw more than doubles the pressure lost getting there, and the pressure at the tap falls as total flow rises. Two hoses off one hose bib commonly give something like 1.3 to 1.6 times the single hose flow rather than 2. Two hoses off separate bibs on separate branches do better. The only way to know is to time the bucket with both running, which is why the calculator asks you to trust a measurement rather than a multiplier.
The other constraint is the source. A well with a small pump and a pressure tank can deliver a burst faster than it can deliver a sustained flow, so a bucket timed in the first thirty seconds overstates what you will get across two hours. If you are on a well, time the bucket after the hose has been running for five minutes.
The weight is the part worth pausing on
Water is 8.34 pounds a gallon, so a seven hundred gallon fill is nearly three tons of water arriving on whatever the tub is sitting on. On a slab that is unremarkable. On a deck it is the whole question, and it needs answering before the hose goes in rather than after, because emptying a tub to fix the structure under it is a much worse afternoon than not filling it. The hot tub deck load calculator takes this volume and adds the tub and the occupants to get the load per square foot.
The same water then has to be heated from whatever came out of the tap, and that is where the fill volume shows up on the electricity bill. Raising 700 gallons by 45 degrees takes about 262,000 BTU, which is roughly 77 kilowatt hours before any losses. The heat loss and cover calculator covers the standing losses that follow, and the pool heater sizing calculator does the same rise-and-time arithmetic on a larger body of water.
Questions people ask
How many gallons is a typical hot tub?
Most residential spas fall between 300 and 600 gallons, with large seven and eight person models running to 700 or more. A two person tub can be under 200. The spread is wide because depth and seat design vary as much as footprint does. If you want a number for your own tub rather than a category average, the specification sheet is the first place to look, and this calculator is the second: measure the water line, measure the depth, and apply a fill factor that matches how much moulded seating is in the shell.
How long does it take to fill a hot tub?
At a common garden hose flow of six gallons a minute, a 500 gallon tub takes about 83 minutes and a 700 gallon tub takes about two hours. Faster supplies cut that substantially and slow ones make it much worse. The only reliable way to predict it is to time a bucket on the hose you will actually use, with any filter you will actually use, and divide the tub volume by the result. Plan to be present for it either way, because an unattended hose in a tub is how people flood a yard or a basement.
Does hot water fill it faster?
It fills at whatever flow the hot line gives, which is often lower than the cold, and it empties your water heater in the process. Filling a several hundred gallon tub from a domestic water heater will exhaust the tank long before the tub is full, and the tub heater still has to bring the whole volume up from the resulting mixed temperature. Some people start a fill with hot water to shorten the heat-up. Whether that is worth doing depends on what heats your water and what it costs relative to the tub heater, and it makes no difference to the fill time unless the hot line is actually faster.
What does the water weigh?
Water is 8.34 pounds per US gallon, so 500 gallons is 4,170 pounds and 700 gallons is 5,838. In cubic feet it is 62.4 pounds each, and there are 7.48 gallons in a cubic foot. The weight matters more than people expect because it never comes off. A deck or a frame under a tub carries the full water weight continuously for as long as the tub is full, unlike almost every other load a residential structure sees.
Can this tell me what to add to the water after filling?
No. Nothing on this site works out a quantity of anything to put in a tub, and that is deliberate. Dosing depends on the tested state of the water, on the concentration of the particular product in front of you, and on what has already been added recently. Getting it wrong affects the people sitting in the water and can damage the equipment. Take quantities from the label of the product you are holding or from someone who can see your test results, test before adjusting, and let the water circulate fully before testing again.