Ordering bacteriostatic water by guesswork produces either stock-outs or a shelf of expired vials. The estimate below takes ten minutes and produces an order that matches how the laboratory actually works.
Step 1: profile each bench
For every workstation that uses bacteriostatic water, write down two numbers: the typical volume per withdrawal and the number of withdrawals per week. Ask the person at the bench; do not guess from the outside. Examples:
| Bench | mL per withdrawal | Withdrawals per week | mL per 28 days |
|---|---|---|---|
| A | 0.2 | 2 | 1.6 |
| B | 0.5 | 4 | 8.0 |
| C | 1.0 | 5 | 20.0 |
| D | 2.0 | 5 | 40.0 |
mL per 28 days = mL per withdrawal × withdrawals per week × 4.
Step 2: assign a vial size per bench
Because an opened vial is discarded at 28 days, the vial should hold about what the bench uses in 28 days, no more. Up to 3 mL: 3 mL vial. 3 to 10 mL: 10 mL vial. Over 10 mL: 30 mL vial. From the table: A gets 3 mL, B gets 10 mL, C and D get 30 mL. Guidance and edge cases are in choosing a vial size.
Step 3: count vials per 28 days
Vials per 28 days = mL per 28 days ÷ vial size, rounded up, minimum one. A: 1 × 3 mL. B: 1 × 10 mL. C: 1 × 30 mL. D: 2 × 30 mL. Monthly total: one 3 mL, one 10 mL, three 30 mL. The vial size recommender does steps 2 and 3 for a single bench.
Step 4: size the order
Multiply by the number of months per order, add a buffer, and round up to pack sizes. For quarterly ordering with a 25% buffer: 3 mL: 1 × 3 × 1.25 ≈ 4 vials, a 4-pack. 10 mL: 4 vials, a 4-pack. 30 mL: 9 × 1.25 ≈ 12 vials, a 12-pack. Check that the last vial in each pack will be opened well before its expiration date; with two-year shelf lives, quarterly ordering is safe.
Adjustments
- Lumpy usage: a bench that uses 20 mL in one week and nothing for a month is better served by 10 mL vials opened as needed than by a 30 mL vial that sits open.
- Many tiny withdrawals: at 0.1 mL per entry, limit punctures per vial by stepping down a size even if the volume fits.
- Single-lot studies: reserve the study's total in one lot as a separate line, outside the routine order.
- Growth: re-run the estimate when benches are added; do not scale the old order.
Turning it into cost
With vial sizes assigned, the cost per millilitre actually used is close to the cost per millilitre bought, because discards are minimized. Compare suppliers and pack sizes on that basis; see what determines the cost per vial.
When the order gets large
Above roughly a case a month, or for a facility with many benches, ask for a standing quote through the bulk and wholesale program. The procurement checklist and the guides for institutional labs and CROs cover the rest.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.