The calculation
Billable seats = the greater of active seats and minimum billable seats. Monthly supplier cost = billable seats × monthly per-seat price + base fee + add-ons + usage + support. Supplier and transition costs = months × monthly supplier cost + setup + external migration + overlap months × old-tool monthly cost + exit reserve.
Internal time = training hours × hourly cost + months × administration hours per month × hourly cost. Total estimate = supplier and transition costs + internal time. Monthly equivalent = total estimate ÷ months. The same hourly assumption applies to training and administration.
Assumptions and excluded inputs
Seats and prices remain constant. Enter monthly-equivalent prices; the result is not an invoice schedule for prepaid annual contracts. The model applies no automatic volume tiers, exchange conversion, taxes or inflation. Incorporate such amounts yourself where appropriate and document the choice. The exit reserve is an allowance, not a confirmed invoice.
Blank fields are explicitly excluded from the estimate, not known zero. Enter zero when that is your deliberate assumption. You can exclude whole cost groups; the results and CSV list those exclusions. A partial estimate must not be mistaken for a complete budget. Currency selection changes the shared unit label only.
Check the sample
For 12 months, 10 seats at 12 per month, 20 monthly add-ons, 100 setup, 200 migration, 4 training hours at 25, 2 monthly administration hours at 25, one overlap month at 50 and a 100 exit reserve, the supplier and transition subtotal is 2,130. Internal time is 700; total is 2,830 and monthly equivalent is 235.83. Other amounts are explicitly zero. These are hypothetical currency-unit amounts.
Read the ownership-cost guide to interpret the estimate. Without scripting, use the formulas above in a spreadsheet, preserve blanks as exclusions and divide only by a positive horizon. Entries stay in page memory and refresh clears them; download your comparison locally to keep it.