Yeast starter calculator

The Yeast Starter Calculator tells you whether the yeast you have on hand — a liquid pack, a dry sachet, or slurry you harvested yourself — meets your target pitch rate for a batch, and if not, how big a starter (or step-up plan) to build to close the gap.

It's a homebrewing reference estimate, not a laboratory cell count, and never valid for a tax or label filing. On the hub it sits in the Estimates (non-regulatory) section, and the calculator opens with a Non-regulatory estimate badge and a highlighted note.

It's free, needs no account, and computes in your browser.

Open the calculator

• On the public site: in the top navigation choose Tools, then — in the Estimates (non-regulatory) section — on the Yeast Starter card choose Open calculator.
• Inside the app: in the navigation choose Tools, then on the Yeast Starter card choose Open calculator.

Find out if you need a starter

1. In Batch volume, enter your batch size and its unit.
2. In Original gravity (OG), enter your wort's starting gravity (or its °Brix equivalent).
3. Choose a Style (Ale or Lager) — this sets the Target pitch rate to the standard convention (0.75 million cells/mL/°Plato for ale, 1.5 for lager, roughly double), which you can edit afterward.
4. Under Yeast on hand, choose the form you have:
   • Liquid pack (smack pack / vial): enter how many packs, how many days since manufacture, and the fresh cell count per pack (commonly 100–200 billion — check your product). Older packs have a viability penalty applied automatically.
   • Dry sachet: enter how many sachets and the grams per sachet (a standard sachet is about 11.5 g). Dry yeast is far more storage-stable, so no age penalty applies.
   • Harvested slurry: enter the volume you collected, its cell density (billion cells/mL), and your estimated viability.
5. Choose a Starter method — Stir plate, Intermittent shaking, No agitation, or None if you don't plan to build one.
6. Cells needed and Cells available appear once every field is filled in, along with the viability applied (for a liquid pack) and whether a starter is needed.

Reading the starter plan

• If your yeast already meets the target, the calculator says so — no starter needed.
• If you're short and picked a method other than None, it sizes a starter: the litres to build and the grams of DME for a standard 1.040 starter wort. A bigger shortfall steps up into two (or more) stages — build the first, let it finish, decant it and keep the yeast, then build the next — with a running total across every stage.
• If you're short and left the method as None, the calculator tells you to pick a method above before it can size anything — there's no starter to plan without one.

Worked example

A 20 L batch of 1.050 ale, pitching a fresh liquid pack with no starter built yet:

1. Batch volume: 20, Unit: Liters (L)
2. Original gravity (OG): 1.050
3. Style: Ale (pitch rate stays at the default 0.75)
4. Yeast on hand: Liquid pack, 1 pack, 0 days since manufacture

The calculator shows Cells needed ≈ 185.8 billion against Cells available 194 billion — a fresh pack alone already covers this batch, so no starter is needed.

A stronger 20 L, 1.080 lager with the same fresh pack falls well short of its (doubled) target, and on a Stir plate the calculator lays out a two-stage step-up plan instead.

When to reach for it

Reach for the Yeast Starter Calculator whenever you're about to pitch and want to check the yeast in your fridge is actually enough — under-pitching is one of the most common causes of a sluggish or off-flavored fermentation. The pitch-rate targets, the liquid-pack viability curve, and the starter growth yields all follow published homebrewing conventions (White & Zainasheff's Yeast; the Braukaiser starter-growth studies) rather than a lab count for your specific yeast, so treat the numbers as planning figures — a note on the calculator says so, and the page-bottom disclaimer confirms it's a reference tool, never a system of record for a filing.