Bars of Pressure: Why 9 Beats 15 (and 20)

If one number sells espresso machines, it’s the bar rating — “15 bar!”, “19 bar!”, “20 bar!” — printed big on the box like a horsepower figure. It’s also the most misleading spec in the category. Espresso is extracted at about 9 bar, and more is not better — it’s often worse. Once you see why, you’ll never be swayed by that number again.

Pump capability is not extraction pressure

Here’s the sleight of hand. The advertised figure is what the machine’s pump can produce, usually unloaded. But the pressure that matters is what your coffee puck actually experiences during the shot — and that should be around 9 bar, the pressure espresso was defined around.

So a “20-bar machine” doesn’t push 20 bar through your coffee. A well-designed one regulates the pressure down to ~9 bar at the puck. The big number describes the pump’s ceiling; it tells you nothing about the shot. Two machines rated 15 and 20 bar can deliver the identical 9 bar to the coffee — the ratings are just different pump specs.

How machines actually hit 9 bar

Good machines control pressure rather than just maxing it out:

• An OPV (over-pressure valve) bleeds off excess pump pressure so extraction settles near 9 bar.
• Some machines add pre-infusion, a gentle low-pressure phase that wets the puck before ramping to full pressure, reducing channelling (the Breville Barista Express does this well).

This is also why an honest spec is “regulated to ~9 bar,” not “up to 20 bar.” When you see the latter, mentally translate it to “our pump is strong,” not “our espresso is better.”

Why more pressure can be worse

Push much past 9 bar and you tend to over-extract and channel: water forces paths through the puck instead of flowing evenly, pulling harsh, bitter flavours. This isn’t hypothetical — the older Gaggia Classic Pro shipped with its OPV set around 11–12 bar, and dropping it to 9 bar is one of the most common and worthwhile first mods owners make. The newer Evo Pro fixed this at the factory. More pressure was the problem, not the feature.

What to actually look for

• Ignore the headline bar number entirely. It’s pump capability, not shot pressure.
• Prefer machines that regulate to ~9 bar (via an OPV) and ideally offer pre-infusion.
• If you buy a machine known to ship with a high OPV, factor in a simple 9-bar adjustment.

Pressure is one of the four specs that actually matter in how to choose an espresso machine — but only the regulated pressure, never the number on the box. The machines in our best espresso machines for beginners all regulate to ~9 bar — exactly what to look for.