What PSI should I set my spunding valve to?
The pressure that holds your target carbonation at the temperature the beer is fermenting at — the same Henry's-law chart every keg calculator prints, read at 64 °F instead of 38 °F. For a 2.5-volume ale fermenting at 64 °F that's 25.6 psi. In a 38 °F kegerator the same carbonation needs 11.2 psi, and that's the number that keeps turning up as spunding advice: Birallee's "10psi", MoreBeer's "12 PSI", Asian Beer Network's "you shouldn't go above 15 PSI". Set 12 psi on a 64 °F fermenter and the beer carbonates to 1.6 volumes, not 2.5 — a flat beer that needs finishing in the keg.
KegLand's guide recommends "8-12psi" for ales, which at 64 °F is 1.5 to 1.6 volumes. That isn't wrong; it's a different job. A valve at 10 psi is controlling esters and catching some gas; a valve at 26 psi is doing the carbonation. Decide which job yours has before you set it. KegLand's own carbonation chart, for what it's worth, agrees with this page's engine within 0.3 psi at every temperature we checked — the chart is right; the "8-12" is about something else.
Carbonation depends on the absolute pressure over the beer; a spunding-valve gauge reads pressure above the local atmosphere. At 5,000 ft the atmosphere is 2.5 psi thinner, so the valve needs 2.5 psi more on the dial for the same carbonation. None of the ranking pages mention it; the calculator adds it when you give it an elevation.
When do you put the spunding valve on?
The rules on the pages that rank for the question:
| Source | Close the valve… |
|---|---|
| BYO, "Advanced Brewing" | "0.004–0.008 above your expected FG" — and, later in the same article, "0.002–0.004 above the FG" |
| Jack's Abby, in BYO | "1 °Plato left in fermentation" (about 4 points) |
| Craft Beer & Brewing | "between two and four gravity points above terminal gravity" |
| Brewer's Friend forum | "3-5 points short of FG"; "about 3 gravity points away" |
| AHA forum | "4 points left, maybe a little more (6) if its an ale" |
| MoreBeer | "On day two" |
| KegLand | "Once fermentation is underway" |
Quoted from each source, 26 Sep 2026.
From "day two" to two points, and BYO gives two different answers in one article. The question has an exact answer, and it's a CO₂ budget: the fermentation still to come has to make the gas to lift the beer from what it already holds to the target, and to pressurize the headspace above it. For the 64 °F ale — 1.050 to 1.010, 19 L in a 27 L fermenter — that's 4.5 points above FG: close at 1.0145.
The forum arithmetic isn't the problem. One gravity point of fermentation near FG makes 0.51 volumes of CO₂ on Balling's balance (46.3 g of gas per 100 g of real extract), so "0.5 volumes per point" holds. What the rule leaves out is that the beer already holds 0.92 volumes at 64 °F, and that the gas has to fill the fermenter's headspace too. Leave out the first and you close too early, harmlessly; leave out the second and you close too late.
Why "3–4 points" works in a keg and not in a FermZilla
Brim-full, with no headspace, the 64 °F ale needs 3.1 points — the forum rule, exactly. Put the same 19 L in a 27 L fermenter and the 8 L of gas above it has to be brought to 26 psi as well: that's 30 % of all the gas the fermentation has to make. In a 35 L vessel it's 46 %, and the answer is 5.8 points. OG barely matters — a 1.040 and a 1.080 beer both land at about 4.5 — but temperature and headspace do:
| Fermenting at | Valve | Full keg | 23 L in 27 L | 19 L in 27 L | 19 L in 35 L |
|---|---|---|---|---|---|
| 50 °F 10 °C | 17.6 psi | 2.6 | 3 | 3.5 | 4.5 |
| 55 °F 13 °C | 20.3 psi | 2.8 | 3.2 | 3.9 | 5 |
| 60 °F 16 °C | 23.1 psi | 3 | 3.5 | 4.2 | 5.5 |
| 64 °F 18 °C | 25.3 psi | 3.1 | 3.7 | 4.5 | 5.8 |
| 68 °F 20 °C | 27.6 psi | 3.2 | 3.8 | 4.7 | 6.2 |
| 72 °F 22 °C | 30 psi | 3.3 | 4 | 4.9 | 6.5 |
Gravity points above FG to close at, for 2.5 volumes; 1.050 → 1.010. Residual CO₂ from the standard fit; gas from Balling's balance on real extract; headspace by the ideal-gas law. Computed by this page's engine.
Too early or too late: which is worse?
Early costs nothing. Close the valve at 1.020 and the fermentation makes more gas than the target needs; the valve does its job and vents the surplus, and the beer ends at the valve's setting. Late is the expensive direction, because the gas that escaped before you closed is gone:
| Closed at | Points above FG | Beer ends at |
|---|---|---|
| 1.016 | 6 | 2.50 volumes — valve vents the rest |
| 1.015 | 5 | 2.50 volumes — valve vents the rest |
| 1.014 | 4 | 2.32 volumes |
| 1.013 | 3 | 1.96 volumes |
| 1.012 | 2 | 1.60 volumes |
| 1.011 | 1 | 1.24 volumes |
The worked case: 64 °F, target 2.5, 19 L in 27 L, valve at 25.6 psi. Computed by this page's engine.
So the calculator's close-at gravity is the latest safe point, and it's only as good as your FG guess — if the beer might finish two points lower than you expect, close two points earlier. That asymmetry only holds with a working spunding valve on the vessel. Seal a vessel with no valve and every surplus point is pressure with nowhere to go, which is the bottle-bomb arithmetic in the stuck-fermentation guide at fermenter scale.
Can my fermenter take the pressure?
The pressure the carbonation needs rises with temperature; the vessel's rating doesn't. Every rating here is from the maker's own page:
| Vessel | Rated to | Holds 2.5 vol up to | At 64 °F, reaches |
|---|---|---|---|
| Corny keg 19 L | 130 psi | any fermentation temperature | 2.5 and beyond |
| FermZilla (PET) | 35 psi | 80 °F | 2.5 and beyond |
| Kegmenter 58 L | 35 psi | 80 °F | 2.5 and beyond |
| Stainless unitank, 15 psi | 15 psi | 45 °F | 1.8 volumes |
Ratings: KegLand (FermZilla "2.5bar (35psi)", Kegmenter "2.5 BAR"), KegLand and MoreBeer Torpedo kegs (130 psi), Ss Brewtech, Spike and Brewtools unitanks (15 psi operating). Computed by this page's engine.
The stainless unitanks are the surprise. Their 15 psi operating rating holds 2.5 volumes only at 45 °F or colder; at ale temperature a full valve gets you 1.8 volumes, and the rest is carbonated cold, after the crash. A FermZilla at 35 psi handles any normal ale target, but a 3.0-volume wheat beer is reachable only to 67 °F.
- Never pressurize a vessel the maker doesn't rate. Buckets and carboys carry no rating. Anvil: the Crucible "is not to be used for pressure fermenting". Spike's Flex is rated to 2 psi, and Ss Brewtech's Chronical vents at 2–2.5 psi.
- The relief valve is not the control. KegLand, on its own red 35 psi PRV: "THIS IS NOT TO BE USED AS SPUNDING VALVE. The pressure rating on these is approximate with tolerance of +/- 10psi." A spunding valve set below the rating must be on the vessel the whole time it's closed.
- PET has an expiry date. KegLand: hydro-test a FermZilla every 24 months, retire it past the date stamp, never pressurize it above 35 °C (95 °F), and keep it out of sunlight.
- Stainless unitanks: the operating pressure is the limit — 15 psi for Ss Brewtech, Spike and Brewtools. The higher "maximum" some also list is not a valve setting.
Does fermenting under pressure really reduce esters?
There's research behind it. BYO cites a study in which raising the top pressure "from 1.05 bars to 1.8 bars (15 to 26 PSI) cut the production of ethyl acetate … and isoamyl acetate … by a lager strain in half at both 50 and 60 °F". That's what KegLand's lower settings are for, and why "lager at ale temperatures under pressure" works for some strains. Two cautions from the other side: Escarpment Labs notes a pressurized fermentation runs "a bit slower", and MoreBeer's author writes that he hasn't "seen scientific data or anecdotal evidence proving" the flavor claims at homebrew scale. The carbonation arithmetic on this page doesn't depend on either.
What happens when you chill it?
Nothing is lost. The gas is dissolved in the beer, so a 2.5-volume beer spunded at 64 °F is still 2.5 volumes at 38 °F — the pressure over it simply drops to 11.2 psi as the cold beer holds the same gas more easily. Transfer under pressure, set the serving regulator to that (the keg carbonation calculator has the chart), and it pours as carbonated as it was planned.
Spunding valve chart (printable)
Valve pressure by the beer's temperature and target, and the points above FG to close at for three setups. Bold settings are past a 35 psi FermZilla; a 15 psi stainless unitank can use only the few at 15 or under.
| Beer temp | 2.0 | 2.2 | 2.4 | 2.5 | 2.6 | 2.8 | 3.0 |
|---|---|---|---|---|---|---|---|
| 50 °F 10 °C | 11.3 | 13.8 | 16.3 | 17.6 | 18.8 | 21.3 | 23.8 |
| 52 °F 11 °C | 12.2 | 14.8 | 17.3 | 18.6 | 19.9 | 22.5 | 25.1 |
| 54 °F 12 °C | 13.1 | 15.7 | 18.4 | 19.7 | 21.1 | 23.7 | 26.3 |
| 56 °F 13 °C | 14.0 | 16.7 | 19.5 | 20.8 | 22.2 | 24.9 | 27.6 |
| 58 °F 14 °C | 15.0 | 17.8 | 20.6 | 21.9 | 23.3 | 26.1 | 28.9 |
| 60 °F 16 °C | 15.9 | 18.8 | 21.6 | 23.1 | 24.5 | 27.4 | 30.2 |
| 62 °F 17 °C | 16.9 | 19.8 | 22.7 | 24.2 | 25.7 | 28.6 | 31.5 |
| 64 °F 18 °C | 17.8 | 20.8 | 23.8 | 25.3 | 26.8 | 29.8 | 32.8 |
| 66 °F 19 °C | 18.8 | 21.9 | 25.0 | 26.5 | 28.0 | 31.1 | 34.1 |
| 68 °F 20 °C | 19.8 | 22.9 | 26.1 | 27.6 | 29.2 | 32.4 | 35.5 |
| 70 °F 21 °C | 20.8 | 24.0 | 27.2 | 28.8 | 30.4 | 33.6 | 36.8 |
| 72 °F 22 °C | 21.8 | 25.1 | 28.4 | 30.0 | 31.6 | 34.9 | 38.2 |
| 75 °F 24 °C | 23.3 | 26.7 | 30.1 | 31.8 | 33.5 | 36.8 | 40.2 |
| Close at, points above FG (2.5 vol) | 50 °F | 55 °F | 60 °F | 64 °F | 68 °F | 72 °F |
|---|---|---|---|---|---|---|
| Full keg | 2.6 | 2.8 | 3.0 | 3.1 | 3.2 | 3.3 |
| 19 L in 27 L | 3.5 | 3.9 | 4.2 | 4.5 | 4.7 | 4.9 |
| 19 L in 35 L | 4.5 | 5.0 | 5.5 | 5.8 | 6.2 | 6.5 |
Gauge psi at sea level; columns are volumes of CO₂. Bold: past a 35 psi FermZilla. Add about 0.5 psi per 1,000 ft of elevation. Close points are the latest safe gravity — closing earlier only vents gas. Never set a valve above the vessel's rating. Standard fit to ASBC solubility data; Balling's balance; ideal-gas headspace. Computed by this page's engine.