Dilution Plating Calculator
Calculate the correct dilution series for plating bacteria or yeast. Determine which dilutions will give countable colony numbers (30-300) based on estimated culture density.
🦠 Cell Biology📐 Volume to plate = target colonies / (CFU/mL × dilution)🧬 Biology
Estimated culture density (CFU/mL)
Target colony count (aim for 30-300)
Volume to plate (mL)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Dilution Plating Calculator | — | Volume to plate = target colonies / (CFU/mL × dilution) | mL |
Step-by-Step Examples
Example 1
Standard plate count
Culture at ~10^8 CFU/mL. Plate 0.1 mL. Want ~100 colonies.
- Dilution needed = 100/(10^8 × 0.1) = 10^-5
- Dilution series: 10^-4, 10^-5, 10^-6
- Plate all three to ensure one gives 30-300 colonies
✓ Use 10^-5 dilution (expect ~100 colonies per plate)
Example 2
Soil sample
Unknown concentration. What dilution series to use?
- Typical soil: 10^6-10^9 CFU/g (dry weight)
- Start at 10^-5 and go to 10^-8
- Always plate at least 3 consecutive dilutions
✓ Plate 10^-5 through 10^-8 for unknown soil samples
Example 3
Recovery experiment
Post-antibiotic kill: expect ~100 survivors/mL.
- Dilution = 100/(100 × 0.1) = 10 fold (1:10)
- Or even undiluted: 100 × 0.1 = 10 colonies expected
- Low survival: plate undiluted or 1:10 dilution
✓ Plate undiluted or 10x dilution for sparse survivors
Real-World Applications
Food Safety Testing
Standardized dilution plating methods (FDA BAM) for foodborne pathogen quantification.
Environmental Microbiology
Enumerate bacteria in soil, water, and air samples. Compare microbial loads across sites.
Clinical Microbiology
Urine culture: plate 1 µL (1:1000 implicit dilution). >10^3 CFU/mL = significant bacteriuria.
Research
Standard viability assay. Antibiotic kill curves, survival assays, spore germination.
Common Mistakes to Avoid
⚠️
Always plate 3 consecutive dilutions
Estimate may be off by 10-100x. Bracketing with 3 dilutions ensures at least one countable plate.
⚠️
Use sterile technique throughout
Contamination gives false colonies. Flame loops, use sterile tips, work near burner or in laminar flow hood.
⚠️
30-300 colony range is critical
< 30 colonies: statistically imprecise (high CV). > 300 colonies: overlap makes counting impossible.
Frequently Asked Questions
Why is the 30-300 rule used? ▾
Statistical: 30 colonies gives ~18% counting error. 300 gives ~6% error. Below 30: too imprecise. Above 300: colonies merge and are indistinguishable.
What dilution should I use for blood samples? ▾
Blood: typically 10-10^4 CFU/mL for bacteremia. Plate concentrated (0.5-1 mL) or use automated blood culture systems. Standard culture detection limit ~1 CFU/mL.
What is spread plating vs pour plating? ▾
Spread plate: add bacteria to surface of solidified agar, spread with glass rod or beads. Pour plate: mix bacteria with molten agar (45°C) and pour. Spread plates better for oxygen-requiring organisms.
How do I do a 10-fold serial dilution? ▾
Add 100 µL culture to 900 µL sterile water (1:10). Vortex. Take 100 µL of this and add to 900 µL (1:100). Repeat. Each transfer is 10x dilution.
Can I use spreadsheet to plan dilutions? ▾
Yes. Create table: tube number, dilution factor, expected CFU/mL, expected colonies if plating 0.1 mL. Adjust until one or two tubes give 30-300 expected.