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A portable toilet has more than one capacity figure. The waste tank holds used contents, while a separate flush-water tank supplies rinsing water. Check both, then plan how often you can empty the toilet and how much weight you can manage.
Table of Contents
Read the correct tank figure
The Thetford specifications list a 21-litre waste tank and 15-litre flush-water tank for the Porta Potti 365. Compare the 365 model on Amazon UK rather than assuming every related Porta Potti has those capacities.
Adding the two numbers does not tell you the usable waste capacity. The tanks do separate jobs, and the lower section must be emptied according to the product instructions.
Estimate use without inventing a day rating
The number of days between emptying depends on the users, how often they use the toilet, the amount of flush water and the required initial preparation. A fixed “family for a weekend” claim cannot account for all those variables.
Use a short trial to learn your actual routine. Check the level indicator where fitted and follow the maker’s emptying guidance. Do not continue using a full tank because an advertised trip duration has not elapsed.
Set a manageable handling limit
A large liquid-filled tank can be heavy and awkward. Water alone weighs approximately one kilogram per litre, which gives a useful sense of scale, though the exact weight of mixed tank contents varies.
You can empty earlier than the maximum capacity. Consider the route to the disposal point, steps and the person who will carry it. The tank’s handle and pour-out position matter as much as its nominal volume.
Plan supplies and access together
Check the designated toilet-waste point and its opening or access arrangements before setting up. Keep the correct preparation products and a suitable refill supply available. Follow their labels for dosing; more additive is not a substitute for timely emptying.
Allow room to detach and remove the tank from the enclosure. Test that movement with the toilet positioned where it will actually be used, leaving the door route clear.
Choose capacity around the routine
A smaller tank may be easier to carry but require more frequent visits to disposal. A larger tank provides flexibility but can encourage waiting until the load becomes awkward. Neither size removes the need for an authorised disposal route.
Buy the capacity you can handle and service reliably. Record the first trip’s refill and emptying pattern, then adjust the schedule. Practical capacity is the amount you can use comfortably within your cleaning and disposal plan.
Continue with our camping wash guide.
Reviewed 10 September 2026. This guide is based on published specifications and guidance.