Cost per wash explained: the maths behind our rankings

Every ranking on this site uses cost-per-use as the value metric. Pence per wash for detergent, pence per tab for dishwasher tablets, pence per 100ml for surface sprays. This guide explains why that number matters more than the price you see on the shelf, how to calculate it yourself, and the household-specific adjustments that change which product is actually right for you.

The mathss

The formula is intentionally simple:

Cost per use = Pack price ÷ Number of uses

For products measured by volume (multi-surface sprays):

Cost per 100ml = Pack price × 100 ÷ total ml

That’s it. There’s no clever marketing here, just division.

Why the shelf price is misleading

The trap with cleaning products is that pack sizes vary wildly within a single brand. Persil Non-Bio is sold in 18-wash, 38-wash, 60-wash, and 130-wash bottles, at very different prices. The unit price on a Tesco shelf gives you the per-litre cost, which is useful, but it doesn’t tell you the cost per actual job.

A 4.55L Persil bottle at £18.99 looks expensive. Divide by 130 washes and it’s 14.6p per wash. The 1L Persil bottle at £8 looks cheap. Divide by 38 washes and it’s 21p per wash, 43% more per wash for the smaller pack.

Same brand. Same product. The bigger pack is the cheaper-per-wash option, and it’s not even close.

Real numbers across our category rankings

The spread between cheapest and most expensive within each category:

• Laundry detergent: 7.5p (Persil 130-wash on S&S) to 22p (Ariel pods) per wash
• Dishwasher tablets: 7.2p (Sun All-in-1 on S&S) to 22p (Finish Quantum) per tab
• Multi-surface spray: 43p (Cif Cream) to 70p (Ecover) per 100ml

Over a year of average UK household use, those gaps come out to £40–80 of savings per category, if you switch from the worst-value option to the best.

Household adjustments

The straight cost-per-wash isn’t always the right number. Three common adjustments:

Hard water households need water-softening detergents or powder formulas. The cheapest liquid won’t perform as well; you’ll re-run washes or hand-treat. Factor in roughly 10–15% more product use.

Sensitive-skin households need non-bio detergents. Non-bio is typically 1–3p per wash more expensive than bio, but it’s the only realistic option if reactions are happening.

Large families save the most from bulk packs because the higher upfront cost is amortised faster. The 130-wash Persil bottle is overkill for a single-person household, it’ll go off before you finish it.

How to apply this to anything

The cost-per-use approach works on any consumable household product, not just cleaning. Toilet roll: cost per roll, not per pack. Kitchen roll: cost per sheet. Shampoo: cost per 100ml. Tea bags: cost per bag.

Once you start thinking in cost-per-use, supermarket multibuys stop being attractive when the per-unit price is higher than the standard pack. They often are.

For the rankings that use this approach in practice, see our best laundry detergent and best dishwasher tablets guides.