How are partial refills calculated?

Enter your starting volume in the current level field. The calculator subtracts this from total capacity to determine remaining volume, then computes time needed to fill the remaining volume.

What conversion factors are used?

Conversions follow NIST SP 811: 1 US liquid gallon = 3.785411784 liters, 1 cubic foot = 28.316846592 liters, and 1 cubic meter = 1,000 liters.

What happens if drain rate matches or exceeds fill rate?

If outflow equals inflow the level never changes. If outflow is larger, the tank drains at the difference, and the calculator shows how long the current volume lasts: 400 gal with 3 GPM in and 8 GPM out empties in 400 ÷ 5 = 80 minutes.

How do I calculate the volume of a cylindrical tank?

Vertical cylinder: V = π × r² × height. In feet, multiply cubic feet by 7.48052 to get US gallons. A 6 ft diameter, 12 ft tall tank holds π × 3² × 12 = 339.3 ft³ = 2,538 gal, so at 4 GPM it fills in 634.5 minutes, about 10 h 35 min. For a horizontal tank that is partly full, the liquid cross-section is r² × acos((r − d) ÷ r) − (r − d) × √(2rd − d²) for depth d, times the length.

How long does it take to fill a 1,000 gallon tank with a garden hose?

Measure your hose with a bucket first, because flow depends on hose size, length and water pressure. At 5 GPM a 1,000 gal tank takes 3 h 20 min, at 10 GPM 1 h 40 min, and at 15 GPM 1 h 7 min. Two hoses on separate taps roughly halve the time if supply pressure holds.

Should I fill a tank to 100%?

Usually not. Most tanks are filled to 90–95% of nominal capacity to leave headroom below the overflow or vent, and fuel tanks need room for thermal expansion. Set the fill target to 90% and the calculator stops the clock there.

How do I find the flow rate of my tap or hose?

Time how long it takes to fill a bucket of known size. If a 10 L bucket fills in 40 seconds, the flow is 15 L/min, so a 1,000 L tank takes about 1,000 ÷ 15 = 67 minutes. Enter that measured rate in LPM.

Can I use barrels or other units?

Convert first. One oil barrel is 42 US gallons, so a 190,000 bbl tank holds 7,980,000 gallons. At 1,500 GPM that takes 5,320 minutes, about 3 days 16 hours 40 minutes.

Why might actual fill times differ from calculations?

This calculator assumes a constant volumetric flow rate. In real plumbing and pump systems, flow rates can decrease as head pressure rises or municipal supply pressure fluctuates.

Tank Fill Time Calculator

Enter tank capacity (or its dimensions), starting level, fill rate and any drain to get fill or empty time in days, hours, minutes and seconds.

Fill time = (tank capacity − volume already in the tank) ÷ (fill rate − any drain rate).

Tank & Flow Inputs

Total Tank Capacity

Starting / Current Volume (optional refill)

Gallons (US)

Fill To (% of capacity)

Fill Flow Rate (per hose or pump)

GPM = gal/min • GPH = gal/hr • LPM = L/min • LPH = L/hr • CFM = ft³/min

Hoses / Pumps Running

Drain / Demand Rate (optional outflow)

Tank Volume From Dimensions

Measure inside dimensions. Liquid depth is optional and gives the current volume.

Tank shape

Diameter

Height

Liquid depth

Unit

Capacity—

Current volume—

100%

Enter tank capacity and fill rate

Visualization illustrates level progression based on steady-state flow, not real-time countdown.

Unit Standards & Calculation Limits

• 1 US Gallon = 3.785411784 L • 1 ft³ = 28.316846592 L • 1 m³ = 1,000 L (NIST SP 811 / HB 44).

• Net Flow Model: Fill duration = (Capacity × Fill % − Current Volume) ÷ (Fill Rate × Hoses − Drain Rate). If the drain wins, time to empty = Current Volume ÷ (Drain Rate − Fill Rate × Hoses).

• Steady Flow Boundary: Actual fill duration can increase as backpressure rises or incoming line pressure drops during continuous operation.

Fill Time Chart by Tank Size and Flow Rate

Time to fill an empty tank at a steady flow, rounded to the minute.

Tank (US gal)	5 GPM	10 GPM	15 GPM	20 GPM	50 GPM
55 55-gal drum	11 min	6 min	4 min	3 min	1 min
100	20 min	10 min	7 min	5 min	2 min
275 IBC tote	55 min	28 min	18 min	14 min	6 min
500	1 h 40 min	50 min	33 min	25 min	10 min
1,000	3 h 20 min	1 h 40 min	1 h 7 min	50 min	20 min
2,500 cistern	8 h 20 min	4 h 10 min	2 h 47 min	2 h 5 min	50 min
5,000	16 h 40 min	8 h 20 min	5 h 33 min	4 h 10 min	1 h 40 min
10,000	1 d 9 h	16 h 40 min	11 h 7 min	8 h 20 min	3 h 20 min
20,000 above-ground pool	2 d 18 h	1 d 9 h	22 h 13 min	16 h 40 min	6 h 40 min

1 GPM = 3.785 L/min = 60 GPH. For liters, 1,000 L at 20 L/min takes 50 min.

Tank Volume Formulas

Vertical cylinder: V = π × r² × h. Partly full: use the liquid depth for h.
Horizontal cylinder: V = L × π × r². Partly full to depth d: V = L × [r² × acos((r − d) ÷ r) − (r − d) × √(2rd − d²)].
Rectangular: V = L × W × H.
Units: 1 ft³ = 7.48052 US gal = 28.317 L; 1 m³ = 1,000 L = 264.17 US gal; 231 in³ = 1 US gal.

Worked Examples

Vertical water tank: 6 ft across, 12 ft tall: π × 3² × 12 = 339.3 ft³ = 2,538 gal. At 4 GPM: 2,538 ÷ 4 = 634.5 min, about 10 h 35 min.
Horizontal fuel tank: 4 ft diameter, 10 ft long holds 940 gal; at 1 ft depth it contains 184 gal. Filling to 90% (846 gal) at 20 GPM takes (846 − 184) ÷ 20 = 33 min.
Filling while drawing water: a 1,500 gal cistern, 300 gal in, 12 GPM in and 2 GPM out: 1,200 ÷ 10 = 2 h.

Quick answer

Fill time = (tank capacity − volume already in the tank) ÷ (fill rate − any drain rate). Keep the units matched, so gallons divided by gallons per minute gives minutes.

Examples

Tank and flow	Calculation	Fill time
1,800 gal at 15 GPM	1,800 ÷ 15 = 120 min	2 hours
3,000 gal, 1,200 in, 15 GPH	1,800 ÷ 15 = 120 h	5 days
5,000 L, quarter full, 833.3 L/h	3,750 ÷ 833.3 = 4.5 h	4 h 30 min

Enter your tank size, current level and flow, including any drain, to get the time in days, hours and minutes.

Summary