How are per-diem charges calculated?

For each investigator, the charge per cage type is: cage_count x rate_per_day x billing_period_days. The tool sums across all cage types for each PI, applies any qualifying volume discount, and produces a net charge. The facility summary totals all PI charges.

How do volume discount tiers work?

You can define discount tiers based on total cage count per PI. For example, PIs with 20+ cages get 5% off, 50+ cages get 10% off. The highest qualifying tier applies. Discounts are subtracted from the gross charge to produce the net charge. Leave tiers empty for flat-rate billing.

What are typical per-diem rates for mouse cages?

Per-diem rates vary widely by institution. Typical US academic rates range from $0.80–$2.00 per cage per day for standard mouse cages. Breeding cages are usually 30–50% more due to larger size and additional monitoring. IVC racks may carry a premium for filter changes. Contact your facility for current rates.

Can I use different rates for different cage types?

Yes. Each cage line within an investigator has its own cage type and rate per day. You can enter standard, breeding, large, and IVC cages at different rates to match your facility's recharge schedule.

ToolsConductScience tool

Vivarium OperationsFree in-browser calculator

Per-Diem Billing Generator.

Generate per-investigator vivarium invoices from cage counts, per-diem rates, and billing period. Supports multiple cage types, volume discount tiers, and fund codes.

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Validated2026-04-08

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Load example per-diem billing generator data to see the full workflow

Billing Period

Period label *

Days in period *

Volume Discount Tiers (optional)

Investigator 1

Name *

Department

Fund code

Cage Lines

Type

Count

$/cage/day

Required fields

Period label is required (e.g., "March 2026").
Investigator 1: name is required.

When to use

Monthly per-diem invoice generation for all PIs
Budget estimation for grant proposals
Evaluating volume discount tier structures
Facility revenue forecasting

Do not use for

Real-time cage tracking (use ConductColony instead)
Per-diem rate calculation (see Recharge Rate Calculator)
Individual cage cost estimation (see Per-Diem Cost Calculator)

Use a Fixed Monthly Census Date

Run billing on a fixed census date each month (e.g., last business day) rather than daily averages — it simplifies disputes and auditing.

Include Fund Codes on Every Invoice

Include fund codes in every invoice to streamline the accounting office's cost allocation workflow.

Account for Different Cage Types

Forgetting to account for different cage types leads to under-billing for breeding cages, which cost more to maintain.

Verify Grant Budget Before Discounts

Applying volume discounts before verifying the PI's grant budget can create allocation issues.

Method

For each PI: line_subtotal = cage_count $\times$ rate_per_day $\times$ billing_period_days. Gross charge = sum of all line subtotals. Volume discount is the highest qualifying tier based on total cage count. Net charge = gross − discount. Facility totals sum across all PIs.

Validated

Last validated 2026-04-08. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.

How to cite

How to Cite

ConductScience Per-Diem Billing & Invoicing Generator (v1.17.0). ConductScience. https://conductscience.com/tools/per-diem-billing-generator

NIH Grants Policy Statement (rev. 2023), Section 7.4: Allowable Costs — Animal Care.

Vivarium Per-Diem Billing Best Practices

Per-diem billing is the standard cost-recovery mechanism for academic and institutional vivaria. Key principles:

Rate setting: Per-diem rates should cover direct costs (bedding, feed, water, cage wash labor, veterinary oversight) plus facility overhead (utilities, equipment depreciation, administrative staff). NIH requires that federally funded animal facilities use rates that do not generate surplus beyond what is needed for operations.

Billing frequency: Monthly billing is standard. Some facilities bill quarterly. The billing period should align with your institution's accounting cycles. Mid-month transfers (cages moving between PIs) are typically prorated or billed to the PI who held the cages on a census date.

Volume discounts: Some facilities offer tiered pricing to encourage efficient colony management. PIs with larger colonies benefit from economies of scale in cage processing. Discounts are typically 5–15% for high-volume users.

Fund code tracking: Each PI may have multiple funding sources (R01, P01, departmental funds). Accurate fund code assignment ensures proper cost allocation and compliance with grant terms.

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