The cycle. Every shuttle is a repeating loop: travel to the fill site, manoeuvre on and connect, fill, manoeuvre off and disconnect, travel back, manoeuvre on and connect, dump into the portable tank, manoeuvre off and disconnect. Add up the minutes of all of it and that is the cycle time. Travel time one way in minutes = 0.65 + 1.7 × distance in miles, which assumes an average of about 35 mph over rural roads including turning and slowing — it is deliberately not the posted speed limit. Handling time at each site is the manoeuvring and connection time plus tank capacity ÷ the rate at that site.

The delivery rate. Tender delivery rate = (usable capacity ÷ cycle time) × k. The 10 % unusable allowance covers the water left in baffled tanks, in the sump, and what sloshes out on the road. The k factor is NFPA 1142's: 1.0 for a vacuum or pressurised tank that discharges under its own power, 0.9 for everything else — gravity dumps into a portable tank, which is slower and never fully empties. Sustainable flow is the single-tender rate multiplied by the number of tenders running, and the number you need is the required flow divided by the single-tender rate, rounded up, because a fraction of a tender does not exist.

Worked example. A 2,500 gallon tender, 3 miles each way at 35 mph, filling and dumping at 1,000 gpm, 2 minutes to make and break at each site. Travel one way = 0.65 + 1.7 × 3 = 5.75 min, round trip 11.5 min. Fill site handling = 2 + 2,500 ÷ 1,000 = 4.5 min; dump site the same. Cycle = 11.5 + 4.5 + 4.5 = 20.5 min. Usable = 2,500 × 0.9 = 2,250 gal. Delivery rate = 2,250 ÷ 20.5 × 0.9 = 98.8 ≈ 99 gpm. To hold 500 gpm you need 500 ÷ 98.8 = 5.06, rounded up to 6 tenders — and four of them, which is what most departments would put on the road, deliver 395 gpm and leave the fire 105 gpm short.

Why the number is optimistic. The travel formula assumes a constant 35 mph with no traffic, no railroad crossing, no bridge, no school zone, no left turn across a highway, and no other apparatus in the convoy. Real cycle times measured in the field routinely come in 20–40 % longer than calculated, and the delivery rate drops in proportion. Fill sites are the usual surprise: a 4" hydrant will not give 1,000 gpm, a static source with a hard sleeve and a small pump will give far less, and the tender that has to wait behind another tender at the same hydrant is not filling at the rated rate at all. Stagger fill sites, mark the turnaround points, and time an actual cycle before you commit a flow figure to the incident commander.

Other things the formula cannot see. Road grade and surface, whether the driver is rested, whether the tender has to be turned around in a field, whether the dump site is a portable tank that overflows if the next tender arrives too soon, and — above all — the requirement. Water supply is only useful against a flow requirement, so calculate what the fire needs first and work backwards from that.

Disclaimer. A planning and training aid — not a substitute for a timed practice run on your own roads, or for your department's water supply SOPs. Sources: NFPA 1142, Standard on Water Supplies for Suburban and Rural Fire Fighting · IFSTA Tanker/Tender Operations · ISO rural water supply methodology. Last reviewed: 2026-09-21.