This transformation efficiency calculator computes cfu per microgram of plasmid DNA — the number that grades competent cells and cloning days — from colonies counted, DNA used, and the fraction of the recovery you plated.

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TE = Colonies ÷ µg DNA Actually Plated

Two fractions hide inside the formula — pg → µg (÷10⁶) and plated ÷ recovery volume — and skipping either inflates efficiency claims by orders of magnitude, the field’s favorite self-deception. Plate-count concentration logic generally lives on our CFU Calculator. Part of our Biology Calculators collection.

Worked Example (Verified)

  • 250 colonies, 10 pg pUC19, 100 µL plated of 1,000 µL recovery (defaults): DNA on plate = 10⁻⁵ µg × 0.1 = 10⁻⁶ µg → TE = 2.5 × 10⁸ cfu/µg — comfortably commercial-grade
  • The forgotten-fraction trap: omitting the plated fraction (claiming all recovery counted) would report 2.5 × 10⁷ — tenfold humbler; claiming µg where pg belonged reports 2.5 × 10², a tragedy. Units are the whole story in this formula
  • Ligation expectations: the same cells on a ligation mix deliver ~10⁶–10⁷ effective TE — plan DNA amounts accordingly instead of declaring the batch dead
  • Electroporation edge: salt in DNA arcs and murders efficiency (and cuvettes) — ethanol-precipitate/desalt before blaming the cells

Troubleshooting the Low-TE Bench

  • Control plasmid first: separates cell competence from construct problems in one plate.
  • Recovery: 45–60 min in rich medium (SOC) post-shock lets resistance express; plating too early reads as zero efficiency.
  • DNA quality: miniprep salt/EDTA carryover suppresses both chemical and electro competence — clean DNA is half the number.
  • Count at the countable density: lawns and 3-colony plates bracket useless; dilution-plate in parallel (Serial Dilution page).

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Frequently Asked Questions

How do you calculate transformation efficiency?

TE = colonies ÷ micrograms of DNA represented on the plate, where plated DNA = total DNA (µg) × (volume plated ÷ total recovery volume). Defaults: 250 ÷ (10⁻⁵ × 0.1) = 2.5 × 10⁸ cfu/µg. Report it with the control plasmid used — TE without the reference DNA is a rumor.

What is a good transformation efficiency?

With supercoiled pUC19: commercial chemically competent cells 10⁸–10⁹ cfu/µg; electrocompetent up to 10¹⁰; competent home preps 10⁶–10⁸. Ligation products score 10–100× lower on the same cells — judge TE against the right class of DNA.

Why is my transformation efficiency zero?

Usual suspects in order: dead selection (wrong/stale antibiotic), no recovery period, DNA issues (too much salt, degraded, wrong amount by 10⁶ — pg vs ng), heat-shock timing/temperature drift, or cells mishandled (freeze-thaw, warming). A pUC19 control plate isolates which system failed in one experiment.

Does more DNA give higher efficiency?

No — TE is per-microgram: excess DNA saturates cells and adds colonies sublinearly, lowering computed efficiency. Tiny inputs (pg of supercoiled control) give the canonical numbers; cloning reactions optimize total colonies, a different objective that may prefer more DNA despite lower TE.

What’s the difference between efficiency and competency checks with ligations?

Controls measure cell quality; ligations measure the product of cell quality × DNA quality × ligation success. Comparing a ligation’s colony count to a control’s TE apportions blame: if control TE is 10⁸ and your ligation plates are empty, the ligation — not the cells — is the crime scene.

Is this transformation efficiency calculator free?

Yes — free, instant, no sign-up.