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Agar Plate Batch Calculator

Plan an agar batch by plate count and recipe. Get exact agar, nutrient, and water weights for MEA, PDA, or honey agar.

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How the formula works

A petri dish is a short cylinder, so per-plate volume is a cylinder volume — π × radius² × depth — converted from mm³ to mL by dividing by 1000 (1 mL = 1 cm³ = 1000 mm³). The total mix volume is then plate count × per-plate volume × (1 + waste factor).

  • Per-plate mL = π × (diameter / 2)² × depth ÷ 1000
  • Total mix mL = plate count × per-plate mL × (1 + waste factor)
  • Agar grams = total mix mL × recipe agar fraction
  • Nutrient grams = total mix mL × recipe nutrient fraction

The waste factor (15% by default) is the chunk you lose to residue clinging to the flask and the last unfillable swirl that's too thick to pour cleanly. Skip it and you'll come up two or three plates short on a 20-plate batch — annoying after you've already sterilized everything.

The three recipes the calculator supports:

  • MEA (Malt Extract Agar): 4% agar + 2% malt extract by weight of total mix. The lab default — every species in the database grows on it.
  • PDA (Potato Dextrose Agar): 2% agar + 4% potato dextrose. Slightly faster mycelial growth, popular for isolations and stock plates.
  • Honey agar: 2% agar + 4% honey. Cheap, available at any grocery store, and works fine for hobby-scale tissue cloning.

Worked example

Target: 20 standard 90 mm petri dishes, 5 mm deep, MEA recipe, 15% waste factor.

  • Per-plate volume: π × 45² × 5 ÷ 1000 ≈ 31.8 mL
  • Total mix: 20 × 31.8 × 1.15 ≈ 730 mL
  • Agar: 730 mL × 4% = 29.2 g
  • Malt extract: 730 mL × 2% = 14.6 g
  • Water: 730 mL (the agar and malt extract dissolve into it; treat the mix volume as the water volume for prep purposes)

Combine in a 1 L flask or jar, leaving headspace, and pressure-cook at 15 PSI for ~20 minutes. Pour at ~50 °C (when the flask is hot but comfortable to handle with a glove) into pre-flame-sterilized petri dishes inside a still-air box or in front of a flow hood. Let plates gel undisturbed for 30 minutes before stacking.

When to use this calculator

Use this calculator any time you're prepping a fresh stack of plates for tissue cloning, multi-spore isolations, or transferring existing stocks to a clean generation. The agar batch is usually the first lab task in a new project, and being one or two plates short because you guessed the volume is the kind of small mistake that wastes an entire pressure-cook cycle.

Why depth matters:5 mm is the sweet spot. Thinner plates (3–4 mm) dry out within a week or two in the fridge — fine if you'll use them immediately, frustrating if you want a stock supply. Deeper plates (6–8 mm) keep longer but waste mix; you'll prep about 50% more agar to fill the same plate count. Most clear PS petri dishes are designed to hold 15–25 mL comfortably, which lines up with 5 mm × 90 mm.

Agar concentration tradeoffs:1.5% agar gives a soft gel that's easy to scoop wedges from but tears under pressure; 2% is the textbook standard and what PDA and honey agar use; 4% in MEA is unusually firm and is what makes MEA wedges hold their shape during agar-to-agar transfers. If you're doing a lot of subculturing, the firmer plate is worth the slightly slower setup.

One frequent substitution mistake: homemade light corn syrup is not a substitute for malt extract. Corn syrup is mostly glucose and fructose; malt extract is mostly maltose plus a small amount of soluble protein and minerals from the barley. Mycelium grows on both, but the nitrogen and trace nutrients in malt extract make MEA meaningfully better for stock plates. Honey agar is closer to MEA nutritionally because honey carries a small amount of pollen and trace minerals; corn syrup carries nothing.

Frequently asked questions

  • MEA vs PDA — which should I use?

    MEA is the better default. It carries a bit more nitrogen than PDA, firmer 4% agar holds wedges together for cleaner transfers, and virtually every wood-loving and dung-loving species in the database grows well on it. PDA is slightly faster for some species and is the standard medium for plant pathology work, but for cultivation, the difference is small and MEA wins on practical handling. If you have both ingredients, use MEA.

  • Can I substitute light corn syrup for malt extract?

    Not really. Corn syrup is mostly glucose and fructose with no nitrogen, while malt extract is mostly maltose plus the trace proteins and minerals that come from barley. Mycelium will grow on corn-syrup agar, but plates dry out faster and growth is noticeably weaker. If you can't source malt extract, honey agar (2% agar + 4% honey) is a much closer substitute — honey carries trace pollen and minerals that corn syrup lacks.

  • Why are my plates pulling away from the dish?

    That's syneresis — the agar shrinking as it dehydrates and contracting away from the petri walls. Three common causes: the plates were poured too thin (under 4 mm), the plates were stored in a low-humidity fridge or near a vent, or the agar concentration was too low (under 1.5%). Pour at the standard 5 mm depth, store stacked and inverted in a sealed bag or container, and you'll usually get 4–6 weeks of useful shelf life.

  • How long do plates keep in the fridge?

    Sealed (with parafilm or a sandwich bag) and stored inverted at ~4 °C, MEA and PDA plates keep 4–6 weeks before drying significantly. Honey agar tends to weep more and is best used within 2–3 weeks. Always inspect a plate before use — any visible mold, color, or off smell means toss it. For longer storage, slant tubes or freezer stocks are the standard; see our agar-pouring tutorial for parafilm sealing technique.

Related resources

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