Design a line, or diagnose one.
Keg and line share the cold space, so the line sits at serving temperature throughout.
What's pushing the beer?
Your serving pressure is whatever holds the carbonation steady at serving temperature — the same equilibrium the priming calculator runs on. Set the carbonation and temperature and it follows, or switch to entering the gauge pressure directly.
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Tubing and geometry.
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How fast should it flow?
This is the knob nearly every calculator hides. The faster the target pour, the more the line has to resist — so the length swings hard with it. A calm pour wants a surprisingly long line.
How long is the run?
Does the line rise steadily?
A beer line should climb in one continuous, gentle rise from keg to faucet — no humps, no sags. At a high point the static pressure drops, CO₂ comes out of solution, and gas collects there between pours; the next pull drives that slug out as foam. A dip traps beer that warms and goes flat. The length math can't see the route the tube takes, so this is on you to get right.
The line to cut
—What each line wants.
Length to balance your target pour at this pressure and geometry, across common tubing. It's why 3/16″ and EVABarrier 4 mm are the workhorses for compact builds, and why 5/16″ barely resists at all.
| Tubing | Inside dia. | Length to balance | Resistance |
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How it's worked out
The real calculation. Pouring beer is pipe flow, so the tool runs it as pipe flow — Darcy–Weisbach for the friction, ΔP = f · (L/D) · ½ρv², with the friction factor f taken from the actual flow: laminar 64/Re below the transition, Blasius 0.316·Re^−0.25 once it's turbulent, which at normal pour speeds it is (Reynolds number around 9,000 in a 3/16″ line). Added to that are the gravity head from the faucet's rise, the restriction of the faucet and fittings as a minor loss K·½ρv², and the velocity head the beer carries out the spout. The applied keg pressure has to cover all of it; balancing the line means sizing the friction term so the sum comes out level at the pour rate you asked for.
Why not the resistance table. The familiar method divides the spare pressure by a fixed “resistance” in psi per foot — 2 ps/ft for 3/16″, and so on. The trouble is that resistance isn't fixed: it rises with flow, because turbulent friction climbs faster than the flow rate itself. A table value is really a snapshot at one unstated pour speed. That's why the old method can hand you five feet of line and a glass of foam — it balances at a faster pour than you meant to use. The static answer is shown here beside the real one so you can watch the two part company; the gap is the flow dependence the table can't carry.
Temperature matters more than people think. Cold beer is more viscous — about 1.7 mPa·s at 38 °F versus 1.4 near 50 °F — so a colder line resists a little more and pours a little slower. Viscosity here follows a standard water correlation nudged up for beer's dissolved solids and alcohol; density is taken as ~1010 kg/m³. Gravity adds back-pressure at the true 0.43 psi per foot of rise, not the rounded half-psi most formulas use.
The shape of the run. The line should rise in one steady, gentle climb. At any high point the static pressure falls by that same 0.43 psi/ft, and because a keg is pressurised only just enough to hold its carbonation, there's almost no headroom before the pressure at a peak drops below what keeps the CO₂ in solution. Gas breaks out, pools at the hump, and launches the next pour as foam. A sag does the opposite — it cradles a slug of beer that warms and de-gasses. Neither shows up in a length calculation, which is exactly why so many “correctly balanced” systems still pour badly.
Scope. This covers home kegerators and keezers, and commercial runs where the line stays inside the cold room at serving temperature. Long glycol-cooled trunk lines — where the beer is warm at the keg, chilled along the run, and the python's own thermodynamics come into play — are a different problem and a later tool. Figures here are a sound starting point; trim a few inches at the faucet to taste once it's pouring.
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