Campervan Waste Tank Size Calculator
Estimate grey-water tank size from your fresh tank and non-drain uses. Get waste per fill, % into tank, and a 20% buffer recommendation.
Why add ~20% on top of the calculated wastewater?
It’s a safety margin for slosh, parking on a slope, and small misestimates. Without headroom, a “just big enough” tank can burp into traps or overflow.
What counts as “non-drain” water?
Anything **not** going to your van’s sink/shower drains: drinking/boiling, a dog bowl, watering plants, outdoor rinse, etc. Add those litres here so the grey-tank math stays honest.
I often top up fresh mid-day — how should I size then?
Think in **multiples of the per-fill waste**. If you add ~50 L mid-day before emptying grey, that’s roughly another **50 L × (share to tank)** you must accommodate.
Does this include black-water (toilet)?
No. This calculator is for **grey water** from sinks/showers only. Toilets need a **separate cassette/composting/black tank** strategy.
My sinks split between inside and outside drains — what do I do?
Estimate the **outside portion** and include it under **Other non-drain use**. The tool assumes anything not marked as non-drain ends up in the grey tank.