How to Calculate Per Diem for a Business Trip

Calculating per diem by hand looks fiddly the first time you try it — a seasonal lodging rate here, a 75% reduction there — but it genuinely comes down to four repeatable steps. Once you've done it for one trip, every trip after that follows the same pattern.

Step 1: Look up the destination rate

Find the GSA lodging and M&IE rates for the city you're visiting on the official GSA per diem lookup tool, or use our own per diem calculator, which pulls from the same underlying dataset. If the city isn't separately listed — and most towns and smaller cities aren't — use the standard CONUS rate: $110 lodging and $68 M&IE for FY2026. Our guide to the standard CONUS rate explains exactly when that fallback applies. Lodging can change by month in seasonal destinations (ski towns, beach resorts, convention cities), so make sure you're using the rate for the specific month you'll actually be travelling, not just an annual average.

Step 2: Count nights and days separately

This is the step people most often get wrong, because lodging and M&IE are counted on different bases. Lodging is paid per night — a trip that arrives Monday afternoon and leaves Wednesday morning is two nights (Monday and Tuesday), not three. M&IE, by contrast, is paid per day, and it includes both travel days — so that same trip has three M&IE days (Monday, Tuesday, Wednesday).

Step 3: Apply the 75% rule to M&IE

Pay 75% of the M&IE rate on the first day of travel and the last day of travel, and 100% on every full day in between. This is a fixed rule under the Federal Travel Regulation, not an estimate — see our full breakdown in the 75% first-and-last-day rule if you want the underlying reasoning and a dollar-by-dollar table.

Step 4: Deduct any provided meals

If any meal was provided free of charge — a hotel breakfast, a conference lunch — subtract that specific meal's published value from the day's M&IE total. The $5 incidentals portion is never deducted, even if all three meals were provided. See are provided meals deducted from per diem? for the exact per-meal figures at each GSA tier.

Step 5: Add it all up

• Lodging = nightly rate × number of nights (using each night's specific month if the rate is seasonal).
• M&IE = (first day at 75%) + (full days at 100%) + (last day at 75%), minus any provided-meal deductions.
• Total per diem = lodging + M&IE.

That example totals $568 for the trip ($330 lodging + $238 M&IE) — a figure that would be easy to overstate by $34 or more if you paid full M&IE on the arrival and departure days instead of the correct 75%.

“Per diem rates for federal travel are set to reasonably reimburse a traveler for lodging and meal costs incurred while on official business, without requiring receipts for every expense.”

— U.S. General Services Administration, gsa.gov

Let a calculator do the day-by-day work

Manually tracking seasonal lodging changes and the 75% rule across a longer, multi-city trip gets error-prone fast. That's exactly what PerDiemWise automates — pick a city and your travel dates, and it returns an itemised, day-by-day total using the current GSA FY2026 figures, ready to attach to an expense report.