Marine Holding Tank Capacity and Pump-Out Planner
Tank and use assumptions
Pump-out plan
Enter the tank and use assumptions to calculate a plan.
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How the planner works
- Use a verified capacity, or calculate a regular tank's gross internal volume and subtract the selected displacement allowance.
- Calculate current contents and the volume at the chosen maximum planning fill.
- Estimate daily inflow from people × uses per person × inflow per use, then add other routed wastewater.
- Divide the space remaining to the limit by daily inflow, and project the entered trip duration.
- Calculate how much must be removed to reach the pump-out target and estimate time only when a delivered flow is supplied.
Formulas and units
Daily inflow = people × uses/person/day × volume/use + other daily inflow
Days to limit = (capacity × fill limit − current contents) ÷ daily inflow
Minimum pre-trip removal = max(0, current contents + trip inflow − fill-limit volume)
Pump time = removal to target ÷ delivered pump-out rate
US gallons, litres, and Imperial gallons are converted through litres: 1 US gal = 3.785411784 L and 1 Imp gal = 4.54609 L. Geometry uses metres internally.
Worked example
A verified 20 US gal tank is 40% full, so it contains 8 gal. With two people, 5 uses each per day, and 0.5 gal entering per use, estimated daily inflow is 5 gal. An 85% planning limit is 17 gal, leaving 9 gal or 1.8 days to the limit.
A three-day trip would add 15 gal and project 23 gal before any pump-out—beyond physical capacity as well as the selected limit. Pumping down to 5% would remove about 7 gal now and leave roughly 3.2 days from that target to the 85% threshold. Actual results depend on measured use, gauge accuracy, tank geometry, and the installed system.
Frequently asked questions
How is holding-tank capacity calculated?
For a rectangular tank, multiply internal length, width, and height. For a round cylinder, multiply π by radius squared and internal length. For an elliptical cylinder, multiply π by half-width, half-height, and internal length. The optional allowance reduces the geometric volume for known internal displacement.
How does the planner estimate days until pump-out?
It subtracts current contents from the selected fill-limit volume and divides that remaining space by estimated daily inflow. If the tank is already at or above the limit, the displayed time is zero.
What inflow per toilet use should I enter?
Use a manufacturer value for the actual flush mode or measure the installed system. Manual heads, electric heads, dry-bowl cycles, hose length, and user behavior can differ enough that a generic value is unreliable.
Should graywater be included?
Include every source physically routed into this tank. In the United States, EPA says graywater is regulated differently, but graywater mixed with sewage must meet sewage-effluent requirements.
What pump-out rate should I use?
Use a measured or facility-provided delivered rate for the complete hose and connection. Leave it blank when unknown; the planner will still calculate removal volume without presenting a time.
Where should a Type III holding tank be pumped out?
EPA says holding tanks must be pumped out at an appropriate onshore or mobile facility. Contact the local marina, marina organization, or responsible authority for facilities and applicable requirements.
Are tank or crew inputs sent anywhere?
No. The calculation runs locally, and this tool does not send, store, or attach input values to analytics events.
Limits and safety disclaimer
- The geometry modes cover only regular rectangular, round cylindrical, and elliptical cylindrical bodies. Hull-following, wedge, stepped, curved-end, or irregular tanks need a drawing, calibration table, or measured fill test.
- A level gauge may not map linearly to volume in an irregular tank, and heel or trim can change the reading. Treat the current percentage as a volumetric estimate only.
- The planner does not model solids, foam, vent blockage, hose volume, incomplete pump-out, leaks, treatment, chemicals, odor control, maintenance, or facility availability.
- Projected contents may exceed tank capacity to show the size of a shortfall; real tanks can overflow or back up. Arrange service before the planning limit.
- Discharge rules vary by waterbody and jurisdiction. In U.S. no-discharge zones, both treated and untreated vessel sewage discharges are prohibited; verify current local requirements for the actual voyage.
Safety and legal disclaimer: This educational estimate does not replace tank documentation, the sanitation-system manual, inspection and maintenance, a facility booking, or advice from the responsible maritime or environmental authority.
Methodology and sources
Last reviewed: August 2, 2026. Geometry uses standard prism and cylinder relationships; liquid conversions and U.S. vessel-sewage context were checked against: