What this calculator works out
This calculator applies a contingency percentage to counts of small, medium and large items, giving a suggested minimum packaging stock.
It estimates quantity only. The packaging type is determined by the evidence characteristics — paper for anything that may carry biological material, sealed containers for volatiles, rigid tubes for sharps — and is not a numerical question. This is a teaching and planning aid, not a casework tool.
Packaging plan
Before you draw stock
- Item counts are an estimate. Scenes reliably produce more exhibits than anticipated, which is what the contingency is for.
- 20% contingency is a reasonable minimum. A complex or unfamiliar scene warrants considerably more.
- Packaging type is set by the evidence, not by size. Damp or biological material requires breathable paper packaging rather than plastic.
- Seals, labels, gloves and a marker are consumed alongside the bags and are equally capable of stopping work.
Sealing damp biological material in plastic traps moisture and promotes bacterial growth and DNA degradation. Paper bags allow drying. Plastic is appropriate for items that must not lose volatile residues, and for wet items only where they are dried under controlled conditions first.
How this calculator works
A contingency applied to each size band:
Stock for each size = estimated item count × (1 + contingency ÷ 100), rounded upEach band is rounded up independently, so the total carries slightly more contingency than the percentage alone implies — which is the right direction for a consumable that cannot be obtained mid-scene.
Worked example: 34 anticipated items
Using the default figures — 20 small, 10 medium and 4 large items, with 20% contingency:
- Small: 24
- Medium: 12
- Large: 5
Forty-one items of packaging for thirty-four anticipated exhibits. The rounding adds a little beyond the 20%, which is deliberate — running out of large bags at a scene means either improvising, which compromises the exhibit, or waiting, which extends the scene examination.
Common mistakes
- Choosing packaging by size alone. The evidence type determines the material.
- Sealing damp biological material in plastic. Use breathable paper packaging.
- Taking no contingency. Scenes produce more exhibits than planned.
- Forgetting seals, labels and gloves. Running out of any of them stops work.
- Re-using packaging. Each exhibit gets its own, sealed and signed.
Frequently asked questions
What packaging is used for what?
Broadly: paper sacks and bags for clothing and anything possibly carrying biological material; nylon or specialist bags for fire debris and volatiles; rigid tubes and boxes for sharps and fragile items; and tamper-evident plastic for items where moisture is not a concern. Local procedure and the laboratory's requirements govern the detail.
How are exhibits sealed?
With a tamper-evident seal signed and dated across the seal, so any subsequent opening is visible. If an exhibit is opened for examination it is resealed with a new seal and the opening recorded, building the continuity record. The number of seals on a package is itself part of its history.
Why does anti-contamination matter so much?
Because modern DNA techniques detect extremely small quantities, transfer between items or from an examiner is a real risk that can be difficult to distinguish from evidential deposition. Single-use gloves changed between items, protective clothing, and separate packaging for every exhibit are all directed at this.
What about wet items?
They are packaged in paper and dried under controlled conditions as soon as practicable, in a drying facility that prevents cross-contamination. Sealing wet items in plastic and leaving them is one of the more damaging things that can happen to biological evidence.
Is what I enter stored?
No. Everything you enter is processed in your browser and is never transmitted or retained. Even so, treat any public website as an unsecured environment and do not enter identifiable case information.
Related tools
References
- GOV.UK — Forensic Science Regulator codes of practice on packaging and anti-contamination
- Health and Safety Executive — handling biological hazards and personal protective equipment
Sources are checked at publication and can change — how I choose and check references.