Spunding calculator
Enter your target carbonation, fermentation temperature and gravity readings to get the spunding valve pressure and the gravity at which to close the fermenter. The CO₂ from the last few points of fermentation then carbonates the beer for you.
Carbonation
While the valve is closed. Warmer beer needs more pressure.
Gravity
Your latest reading.
Set the valve to
26.1 psi
- Close the valve at · 6.8 points above expected FG
- 1.017 SG
- To go from your reading
- 3.2 points
- Time until you close it · rough
- ~8 hours
- CO₂ already in the beer
- 0.86 vol
- Extract for the CO₂ target
- 2.8 points
- Safety margin
- 4.0 points
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How the spunding calculator works
Spunding means closing the fermenter or keg with a spunding valve before fermentation is finished. The yeast keeps turning the remaining extract into alcohol and CO₂, and instead of bubbling out of the airlock, the CO₂ stays in the beer. The valve vents anything above the pressure you set.
The beer already holds some CO₂ from fermentation, more the colder it is. The calculator subtracts that residual CO₂ from your target, converts the rest into gravity points at 0.54 vol of CO₂ per point and adds your safety margin. Add those points to your expected final gravity and you get the trigger gravity: the reading at which you close the valve.
SG_spund = FG + (P_CO₂ + M) / 1000
P_CO₂ = (V_target − V_rest) / 0.54
V_rest = 3.0378 − 0.050062·T + 0.00026555·T²
- SG_spund
- Gravity at which you close the valve
- FG
- Expected final gravity (SG)
- P_CO₂
- Gravity points needed for your CO₂ target
- M
- Safety margin in gravity points (default 4)
- V_target
- Target CO₂ in volumes
- V_rest
- CO₂ already in the beer at temperature T
- T
- Beer temperature in °F
One gravity point is 0.001 SG. With the defaults (2.4 vol, 20 °C / 68 °F, FG 1.010) you close the valve at about 1.017. You can enter gravities in SG or °Plato.
Spunding valve pressure and temperature
How much CO₂ stays dissolved depends on two things: pressure and temperature. Warm beer holds less CO₂, so it needs more pressure for the same carbonation. For 2.5 vol a beer at 4 °C (39 °F) needs about 12 psi, at 20 °C (68 °F) more than twice that.
The calculator uses the same pressure formula as common keg carbonation charts. Set the valve to the pressure shown for the temperature the beer will sit at while the valve is closed. If that temperature changes, the right pressure changes with it, which is what the chart under the result is for.
psi = −16.6999 − 0.0101059·T + 0.00116512·T² + 0.173354·T·V + 4.24267·V − 0.0684226·V²
- T
- Beer temperature in °F
- V
- Target CO₂ in volumes
| Temperature | 2.2 vol | 2.5 vol | 2.7 vol |
|---|---|---|---|
| 46 °F / 8 °C | 12.0 | 15.6 | 18.0 |
| 50 °F / 10 °C | 13.8 | 17.6 | 20.1 |
| 54 °F / 12 °C | 15.6 | 19.5 | 22.1 |
| 57 °F / 14 °C | 17.4 | 21.5 | 24.3 |
| 61 °F / 16 °C | 19.2 | 23.5 | 26.4 |
| 64 °F / 18 °C | 21.0 | 25.6 | 28.6 |
| 68 °F / 20 °C | 22.9 | 27.6 | 30.8 |
When to spund: trigger gravity and safety margin
Close the valve too late and there is not enough extract left to reach your carbonation. Close it too early and nothing bad happens: the valve vents the extra CO₂. That is why the calculator adds a safety margin, 4 gravity points by default. It covers the CO₂ needed to pressurize the headspace, small leaks and a final gravity that ends up a little different from your estimate.
Enter your original gravity, the current reading and the days since pitching, and the calculator also gives a rough time until you reach the trigger gravity. It extrapolates the average rate so far. Fermentation slows down towards the end, so treat it as a hint and check with a reading.
- Only spund in vessels rated for pressure, such as kegs and pressure fermenters. Never in glass carboys or plastic buckets.
- Set the valve before you close it, and check that the vessel’s pressure relief valve works.
- Base the final gravity on your yeast and recipe, or on a forced fermentation test.
- Missed the point? Close the valve anyway and top up with CO₂ or force carbonate later.
Spunding in German lager brewing
Spunding is traditional German lager practice. The young beer goes to the lagering tank with a little extract left, often around 1 °Plato (about 4 gravity points) above final gravity, and the tank is closed with a spunding device. The last of the fermentation carbonates the beer under pressure, without priming sugar or a CO₂ bottle.
At lager temperatures you need much less pressure. For 2.5 vol a beer at 10 °C (50 °F) needs about 17.6 psi (1.2 bar), at 20 °C (68 °F) about 27.6 psi (1.9 bar). Cold beer also holds more residual CO₂, so fewer gravity points are needed and the trigger gravity sits closer to final gravity.
- Enter the temperature the beer is at while the valve is closed, not your serving temperature.
- Ales work too: spund in the last days of fermentation at ale temperatures with a higher valve setting.
- Transfer the carbonated beer under pressure so the CO₂ stays in it.
Built into every BrewForge recipe
In BrewForge you switch on spunding for a primary, secondary or tertiary stage of your fermentation profile and set the pressure. BrewForge takes the stage temperature and your recipe’s planned final gravity, calculates the trigger gravity with the same model as this calculator and projects from your readings when the beer will reach it. Readings can come from manual entries or from an iSpindel, Tilt, RAPT Pill or GravityMon.
- Spunding pressure per fermentation stage
- Adjustable safety margin, 4 gravity points by default
- Spunding forecast on the brew’s fermentation tab, in the calendar and in your ICS feed
Use this calculator via API
The same calculation is also a free JSON API: no account, no key, one GET request. Handy for scripts, spreadsheets and AI assistants that should use exact brewing math instead of estimating it.
Add units=imperial (or one unit parameter per value) for °F, gallons and pounds. Every answer lists the inputs, the units and the formula it used.
FAQ
Questions & answers
When should I close my spunding valve?
When the gravity reaches the trigger gravity: your expected final gravity plus the points needed for your CO₂ target plus a safety margin. For a typical 2.4–2.5 vol beer that is about 6–7 gravity points above final gravity with the default safety margin, depending on temperature. The calculator gives you the exact value.
What pressure should I set on my spunding valve?
It depends on your target carbonation and the beer temperature. For 2.5 vol you need about 17.6 psi (1.2 bar) at 10 °C (50 °F) and about 27.6 psi (1.9 bar) at 20 °C (68 °F). Enter your values in the calculator for the exact setting.
What if I missed the spunding point?
Close the valve anyway to keep the CO₂ that is still being produced. The beer will end up below your target, so top it up later by force carbonating in the keg. The calculator warns you when your current gravity is already below the trigger gravity.
Can I spund in any fermenter?
No. Spunding builds up real pressure, often 1–2 bar (15–30 psi). Only use vessels made for it, such as corny kegs or pressure fermenters, with a working pressure relief valve. Glass carboys, plastic buckets and standard fermenters are not safe under pressure.
Why add a safety margin?
Closing a little early costs nothing because the valve vents the extra CO₂. Closing too late means the beer stays under-carbonated. The margin covers the CO₂ that fills the headspace, small leaks and a final gravity that differs from your estimate. BrewForge uses 4 gravity points by default.
Does spunding only work for lagers?
No. It is traditional for German lagers, but it works for any beer. Warm ale fermentations need a higher valve pressure for the same carbonation, so set the valve to the value for your actual fermentation temperature.
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