Reverse Osmosis vs Carbon: Which Do You Need?

The two most common under-sink filtration technologies are carbon block filters and reverse osmosis (RO). They work in fundamentally different ways, remove different contaminants, and suit different budgets and kitchens. This guide explains the difference so you can choose with confidence.

How each technology works

Carbon block filters pass water through a dense block of activated carbon. The carbon's enormous surface area adsorbs chlorine, volatile organic compounds and compounds that cause bad taste and odour. Carbon does not remove dissolved minerals — so it will not soften hard water or reduce limescale.

Reverse osmosis forces water under pressure through a semi-permeable membrane with pores roughly 0.0001 microns across. That is small enough to reject dissolved solids, including sodium, lead, fluoride, nitrate and the calcium and magnesium that cause hardness. RO systems typically include a carbon pre-filter and a post-filter, so they also remove chlorine and taste.

Side-by-side comparison

Which should you choose?

Choose carbon if your main complaint is chlorine taste or odour, your water is soft to moderately hard, and you want the simplest, most compact, lowest-cost system. Carbon is the right call for most of Scotland, Wales and the South West. See our under sink water filters.

Choose reverse osmosis if you have very hard water and want to remove limescale at the tap, you are concerned about lead from old pipes, or you want to reduce fluoride and dissolved solids. RO is the right call for London, the South East and East Anglia. See our reverse osmosis systems, or read the RO-300 vs RO-400 vs RO-500 comparison if you already know RO is right and just need to pick a model.

If you are in a hard water area but do not need the deep filtration of RO, a dedicated limescale filter paired with a carbon cartridge is a cost-effective middle ground.