Why should I make a yeast starter for beer?

A yeast starter increases cell count and vitality before pitching into wort. This reduces lag time, lowers the risk of contamination, improves attenuation, and leads to cleaner, more predictable fermentations, especially for higher gravity beers or older yeast packs.

How long should a beer yeast starter run on a stir plate?

Most homebrewers run a starter on a stir plate for 18–24 hours at fermentation temperature. Once you see a dense layer of yeast and activity has slowed, you can cold crash to settle the yeast, decant the spent wort, and pitch the slurry.

What DME to water ratio should I use for a yeast starter?

A common rule of thumb is 100 grams of light DME per liter of water, which yields a starter wort of about 1.035–1.040 specific gravity. This is strong enough to promote growth without stressing the yeast.

Can I use this calculator for sourdough starter?

Yes. Switch to the Sourdough mode to calculate flour and water amounts for common feeding ratios (1:1:1, 1:2:2, etc.), hydration percentage, and total starter weight. It is not a fermentation time predictor, but it helps you size feedings accurately.

Do I need a stir plate to make a yeast starter?

A stir plate is not required but it increases oxygenation and keeps yeast in suspension, which can roughly double growth compared to a static starter. Without a stir plate, swirl the flask vigorously whenever you walk by to introduce oxygen.

Yeast Starter Calculator

Plan beer yeast starters and sourdough feedings in one place. Get precise DME, water, and step-up volumes for homebrew, plus flour and water ratios for sourdough starter.

Beer Yeast Starter Planner

Batch volume

Original gravity (OG)

Yeast type

Yeast source

Yeast age

days

Starter method

Starter steps

Max flask volume

Actual starter volume will be kept below ~80% of this.

Starter gravity (SG)

Typical range 1.035–1.040.

Sourdough Starter Feeding Calculator

Current starter

Feeding ratio

Target total starter

Enough for your recipe plus a bit extra.

Flour hydration

How this yeast starter calculator works

This tool combines a practical homebrew yeast starter calculator with a sourdough starter feeding planner. It is designed for real-world use: simple inputs, sensible defaults, and clear, step-by-step outputs.

Beer yeast starter model

For beer, the calculator estimates how many yeast cells you need, how many you currently have, and how large a starter (or two-step starter) you should make to hit a healthy pitch rate.

1. Convert OG to degrees Plato (°P)

We approximate °P from specific gravity (SG):

°P ≈ −463.37 + 668.72 × SG − 205.35 × SG²

2. Required cells

Pitch rate depends on style:

Ale: 0.75 million cells per mL per °P
Lager: 1.5 million cells per mL per °P
Hybrid: 1.0 million cells per mL per °P

Required cells:

cells_required = pitchRate × volume_mL × °P

Viable cells in your yeast pack are estimated from a typical starting count and an age-based viability curve.

3. Yeast viability estimate

We assume:

Liquid pack: 100 billion cells fresh, ~0.7% loss per day
Dry yeast: 200 billion cells per 11 g pack, ~0.2% loss per day
Slurry: user-provided volume is treated similarly to a liquid pack (approximation)

viability ≈ max(0, 1 − decayRate × ageDays)

cells_viable = cells_fresh × viability

Starter growth depends on volume, gravity, and agitation. We use conservative growth factors inspired by lab data and common homebrew calculators:

Stir plate: up to ~4× growth for a well-sized starter
Intermittent shaking: ~2.5× growth
Static: ~1.5× growth

The tool caps starter volume at about 80% of your flask size and suggests a second step if needed.

DME and water amounts

Starter wort is sized using the classic rule of thumb: 100 g of light DME per liter of water for ~1.037 SG.

DME calculation

For a target starter gravity SG_starter and volume V_L:

DME (g) ≈ 100 × V_L × (SG_starter − 1) / 0.037

At the default 1.037, this simplifies to ≈ 100 g per liter.

Sourdough starter mode

The sourdough mode is a flexible feeding calculator. You choose how much starter you currently have, the feeding ratio, and how much total starter you want. The tool then tells you exactly how much starter to keep and how much flour and water to add.

Feeding ratio

Let the ratio be S : F : W (starter : flour : water).

Total parts = S + F + W

For a target total weight T:

starter kept = T × S / (S + F + W)
flour added = T × F / (S + F + W)
water added = T × W / (S + F + W)

Hydration (%) = 100 × water / flour.

Step-by-step: making a beer yeast starter

Calculate your starter. Enter batch size, OG, yeast type, age, and method. Note the suggested starter volume and DME.
Prepare the wort. Mix the calculated DME with water, bring to a boil for 10 minutes, and cool to fermentation temperature.
Sanitize everything. Flask or jar, foil or stopper, stir bar, funnel, and scissors for the yeast pack.
Pitch and aerate. Add yeast to the cooled starter wort and place on a stir plate, or swirl frequently.
Grow and crash. Let it run 18–24 hours, then refrigerate to settle the yeast. Decant most of the spent wort before pitching.

Common pitfalls to avoid

Using too small a starter for high gravity beers.
Pitching hot starter wort into cold yeast (or vice versa).
Skipping sanitation because “it’s just a starter”. Contamination here goes straight into your main batch.
Letting the starter ferment completely out and then waiting days before pitching (yeast will go dormant and lose vitality).

FAQ

Can I make a starter with dry yeast?

Most dry yeast manufacturers recommend rehydrating and pitching directly rather than making a starter, because starters can deplete the built-in nutrient reserves. However, for very high gravity beers or older packs, some brewers still use small starters. If you follow manufacturer guidance, you can set yeast source to “dry” and use the calculator only to check if you need more packs.

How big should my flask be?

A 1 L flask is fine for small ales up to ~1.050. For bigger beers or lagers, a 2 L or 3 L flask is more comfortable. This calculator keeps starter volume below about 80% of your flask capacity to reduce boil-over and foaming risk.

How often should I feed my sourdough starter?

At room temperature, many bakers feed once or twice per day. Higher feeding ratios (like 1:3:3 or 1:4:4) slow the fermentation and can stretch the time between feedings. Use the calculator to size feedings so you have enough active starter when your dough is ready to mix.