Putting a number on bypass: the percentage of total brew water that didn't pass through the coffee bed — water that fell down the side of the filter, or water added directly to the served cup, never touching the slurry. A 0 % bypass brew has every drop interact with the grounds. A 30 % bypass brew has 30 % of its water arrive at the cup as plain hot water that just dilutes the rest.
Why the second number matters
A standard recipe gives you one ratio: coffee to water. 1:16 means 15 g of coffee to 240 g of water. But strength in the cup depends on how much of that water actually extracted coffee.
- No bypass at 1:16 → cup tastes like a 1:16 brew.
- 20 % bypass at 1:16 → cup tastes closer to a 1:20 brew, even though you used the same dose and the same water volume.
Don't confuse this with Tetsu Kasuya's 4:6 method, which splits the water for a different reason: there the last 60 % sets strength through how many pours you break it into, not through water skipping the bed. Both are ways to move strength without touching the dose — but 4:6 works by changing extraction, bypass works by diluting it.
How to estimate yours
You can't measure bypass directly without lab equipment, but you can read it backwards from the cup. Brew the same recipe two ways: a careful centred pour-over and a sloppier wall-hitting pour-over. Same dose, same water, same time. The sloppier brew tastes thinner — that's the difference bypass made.
For most home pour-overs, bypass sits somewhere between 5 % and 25 % depending on technique and brewer geometry. Knowing it exists turns "this cup is weak" into "I let too much water bypass."
