A draught system has one job: get carbonated beer from the keg into the glass at a sensible speed, without losing the CO₂ you worked hard to put in. The keg has to sit at a pressure high enough to keep the CO₂ dissolved. If nothing slowed the beer down on its way out, that pressure would fire it out of the tap as a glass of foam. The beer line is the brake. Its length is chosen so that it soaks up exactly the pressure left over, and the beer arrives at the tap gently.
This is called balancing a draught system, and it comes down to a single pressure budget:
That's why the calculator starts with carbonation: once you know your serving pressure, everything else is about getting rid of it. We can't change Pkeg without changing the beer, and the rise and fittings are set by how the system is built. That leaves the line to absorb whatever is left. The calculator works out each part from first principles, then solves for the length of line that makes the budget add up.
CO₂ dissolves in beer according to Henry's law: at a fixed temperature, the amount of gas dissolved in a liquid is proportional to the pressure of that gas above it. Double the CO₂ pressure and, once things settle, you double the dissolved CO₂.
The catch is that kH depends strongly on temperature. Cold liquid holds far more gas: pure water at 1 atmosphere dissolves roughly 1.7 volumes of CO₂ at 0 °C but only about 0.9 volumes at 20 °C. That is why warm kegs need much higher pressure for the same fizz, and why a keg that warms up will push CO₂ out of solution and pour foamy.
Beer is not pure water (alcohol, sugars and proteins all nudge solubility), so instead of a theoretical kH the calculator uses a regression fitted to the industry standard carbonation table published by Zahm & Nagel. With T in °F and V in volumes of CO₂:
A regulator gauge reads pressure above the surrounding air (gauge pressure), but Henry's law cares about absolute pressure. At altitude there is less air pushing down, so the gauge needs to read a little higher to reach the same absolute pressure. The calculator estimates local air pressure with the standard atmosphere formula and adds the difference. For Perth and anywhere near sea level, this correction is zero.
Carbonation is measured in volumes of CO₂: the volume the dissolved gas would take up as a gas at 0 °C and normal air pressure, divided by the volume of beer. A beer at 2.5 volumes holds 2.5 litres of CO₂ in every litre of beer. The pressure column below updates to match the serving temperature you enter in the calculator.
| Style | Typical vol CO₂ | Serving pressure |
|---|---|---|
| British bitter, mild and ESB | 1.5 to 2.0 | -- |
| Porter and stout | 1.7 to 2.3 | -- |
| Brown and amber ale | 1.8 to 2.5 | -- |
| American pale ale and IPA | 2.2 to 2.7 | -- |
| Australian pale ale and lager | 2.5 to 2.8 | -- |
| German lagers (helles, pilsner, märzen) | 2.4 to 2.7 | -- |
| American lager | 2.5 to 2.8 | -- |
| Belgian ales (blonde, dubbel, tripel) | 2.5 to 3.5 | -- |
| Saison | 2.8 to 3.5 | -- |
| German wheat beer (hefeweizen, dunkelweizen) | 3.3 to 4.5 | -- |
| Sours (Berliner weisse, gose, gueuze) | 2.8 to 4.0 | -- |
| Sparkling cider | 2.4 to 3.0 | -- |
These ranges are brewing tradition and taste, not chemistry. Published charts vary by a few tenths of a volume, and plenty of great beers sit outside them. Pick a starting point, pour a few glasses and adjust. Two things worth knowing:
High carbonation needs high pressure, and high pressure needs a lot of line to absorb it. Wheat beers and saisons often end up on their own longer line or a flow control tap.
Nitro stouts are served with a nitrogen and CO₂ blend through a stout tap with a restrictor plate. This calculator assumes straight CO₂, so use the porter and stout row only for stouts served on normal CO₂.
Pushing beer uphill takes pressure, exactly like pumping water to the top of a tank. This is hydrostatic head:
For beer at SG 1.010 that works out to about 9.9 kPa (1.44 psi) per metre of rise. If your tap sits below the keg, h is negative and gravity helps you, so you need a little more line, not less.
Technically the height should be measured from the beer surface inside the keg, which drops as the keg empties. Measuring from the middle of the keg splits the difference.
This is the heart of the calculation. First we need the speed of the beer in the line, which comes from your pour rate and the inside area of the tube:
Next, the Reynolds number tells us how the beer is flowing. Below about 2,300 the flow is laminar: smooth, orderly layers sliding past each other. Above about 4,000 it is turbulent: swirling and chaotic, and much more resistant. In between is a transition zone.
Here's the surprise for most brewers: at normal pour rates, beer in a 3/16" line is turbulent, usually with a Reynolds number around 5,000. That matters because the simple laminar formula (Hagen–Poiseuille) would badly underestimate the resistance.
Pressure lost to friction in any pipe is described by the Darcy–Weisbach equation:
The friction factor f depends on the Reynolds number. The calculator uses the Churchill equation, a single formula that is accurate across laminar, transitional and turbulent flow, so you never have to pick the right one yourself. In turbulent flow, f is roughly 0.316 / Re0.25 (the Blasius relation), which you can use to check the numbers by hand.
Viscosity starts from water's viscosity at your serving temperature (calculated with the Vogel equation) multiplied by the viscosity factor. Finished beer is typically around 1.4 to 1.8 times as viscous as water because of its dissolved dextrins, proteins and alcohol, so we default to 1.6. Don't stress about the exact number: in turbulent flow, line resistance only scales with viscosity to the power of 0.25, so a 25% error in viscosity shifts the line length by only about 6%.
Every change of direction or diameter costs a little pressure, and the beer still carries some energy as it leaves the tap. Engineers bundle these into a loss coefficient, K:
The exit alone is worth K = 1 (the beer leaving the line carries away its full dynamic pressure). The coupler, dip tube entry, barbs, shank and tap add roughly another 1 between them, so we default to K = 2. This is an estimate, not a measured value for your hardware, which is why it sits in the fine tuning section.
This also explains an old rule of thumb. Many balancing guides say to leave "about 1 psi at the tap". At a typical pour, ρv²/2 is around 3 to 6 kPa (0.5 to 0.8 psi), so K = 1 to 2 lands right on that 1 psi rule. The rule is physics in disguise.
The panel under the calculator shows every one of these steps with your own numbers, so you can follow the working through or check it with a pen and paper.
Beer at 4 °C and 2.5 volumes, 3/16" line, tap 30 cm above the middle of the keg, pouring at 45 mL/s:
Pkeg = 81.8 kPa (11.9 psi)
v = 2.53 m/s, Re ≈ 4,950 (turbulent), friction = 26.1 kPa per metre
Prise = 3.0 kPa, Pfittings = 6.5 kPa
L = (81.8 − 3.0 − 6.5) / 26.1 = 2.78 m
Same system, but the hefeweizen is carbonated to 3.5 volumes. The keg now needs about 155 kPa (22.5 psi), so there is far more pressure to soak up:
L = (155.1 − 3.0 − 6.5) / 26.1 = 5.59 m
This is why high carbonation styles are often served on their own longer line, or on a flow control tap.
Beer at 4 °C and 2.5 volumes, 4 mm ID line, 60 cm rise, pouring at 45 mL/s. The narrower line makes the beer travel faster (3.58 m/s), so friction jumps to 59.2 kPa per metre:
L = (81.8 − 5.9 − 13.0) / 59.2 = 1.06 m
Narrow lines balance in much shorter lengths, which is handy in a small fridge.
Most balancing guides use a fixed resistance per foot of line, such as 3 psi per foot for 3/16" vinyl. Those figures come from commercial draught guidance and assume a fast commercial pour of about 1 US gallon per minute (roughly 63 mL/s). Plug the pale ale example above into that method and you get a line of only about 1 m.
The physics shows why that comes up short. Line resistance is not a fixed property of the tube. In turbulent flow it rises with roughly the 1.75 power of the pour rate, so a gentler homebrew pour means much less resistance per metre and a much longer line. Even at a commercial pour rate, the Darcy–Weisbach working gives noticeably less than 3 psi per foot for smooth tubing. This lines up with what many homebrewers find in practice: lines cut to the 3 psi per foot rule pour foamy, and lines of around 3 m are common for 3/16" tubing at normal serving pressures.
The calculator shows the rule of thumb result alongside the first principles result (where a published figure exists for your tubing size) so you can see the difference for yourself.
The science gets you close. These practical habits get you the rest of the way:
Cut your line 10 to 15% longer than the calculated length. You can always trim 10 cm at a time until the pour is right, but you can't add line back.
Everything here assumes the beer stays at serving temperature all the way to the tap. A warm tower or shank lowers CO₂ solubility right where pressure is dropping, and that's where foam is born. Insulate or cool your tower if you can.
If a balanced line pours too fast, don't drop the regulator pressure to slow it. The beer will slowly lose carbonation to match the lower pressure. Lengthen the line or fit a flow control tap instead.
Biofilm and beer stone roughen the inside of the line and create spots where CO₂ breaks out. Clean your lines regularly with a proper line cleaner.
This calculator assumes the beer is fully carbonated to the target level, stays at one temperature from keg to tap, and flows through smooth plastic tubing of a single diameter. The carbonation equation is fitted to tabulated data and is most reliable between roughly 0 and 25 °C. The viscosity factor and fittings loss coefficient are reasonable estimates rather than measurements of your gear, so treat the result as an excellent starting point and fine tune by pouring.
