What this TDY per diem calculator does
Temporary duty travel often comes with a per diem entitlement intended to reimburse reasonable travel costs while you are away from your permanent duty station. In many travel programs, per diem is made up of two components: lodging (reimbursed up to an authorized nightly cap) and meals & incidental expenses (M&IE) (a flat daily amount intended to cover food, tips, and small incidentals). Because rates vary by location and because the first and last travel days are commonly paid at a reduced percentage, it helps to estimate your entitlement before you book lodging, compare trip lengths, or prepare a voucher.
This page provides a simplified TDY per diem calculator using a small set of sample domestic rates. It is designed for planning and learning: you can quickly see how trip length affects lodging nights, how M&IE is applied across travel days, and what the combined total looks like. The results are not official and are not a substitute for your travel orders, your organization’s policy, or the published per diem tables.
How to use the calculator
- Select a destination from the Location list. Each location has a sample lodging cap and a sample M&IE rate.
- Enter travel days as the total number of calendar days you are away, including the departure day and the return day.
- Click Calculate Per Diem to generate a breakdown table showing lodging, M&IE, and the total estimate.
- Use Copy Summary to copy the results into a note, spreadsheet, or voucher worksheet.
Planning tip: If you are comparing options (for example, a 3-day conference versus a 7-day course), keep the location the same and change only the number of days. You will immediately see the impact of additional lodging nights and full M&IE days. If you are comparing locations, keep the days the same and switch the destination to see how much of the difference comes from lodging versus M&IE.
Formula and assumptions
The calculator follows a common simplified approach that matches the behavior of the script on this page. It is intentionally straightforward so you can understand the mechanics.
- Lodging nights are assumed to be travel days − 1. For example, a 3-day trip typically has 2 nights of lodging.
- M&IE is paid at 75% on the first and last travel day, and at 100% on the days in between.
- The estimate assumes no meals are provided at government expense (no meal deductions are applied).
- All rates shown are sample values for demonstration and do not update seasonally.
Using variables: L = lodging rate (per night), M = M&IE rate (per day), d = travel days.
Nights = max(d − 1, 0)
Lodging total = L × Nights
Full M&IE days = max(d − 2, 0)
M&IE total = (M × Full M&IE days) + (0.75 × M × min(d, 2))
Total per diem = Lodging total + M&IE total
In plain language: the calculator counts one fewer lodging night than the number of travel days, then applies the 75% M&IE rule to up to two travel days (the first and last). Any remaining days in the middle are paid at the full M&IE rate.
Worked example (step-by-step)
Example scenario: You travel to San Diego, CA for 3 days. The sample rates in this calculator are $182 lodging and $79 M&IE.
- Nights = 3 − 1 = 2 nights
- Lodging total = 2 × $182 = $364.00
- Full M&IE days = 3 − 2 = 1 full day
- M&IE total = (1 × $79) + (0.75 × $79 × 2) = $79 + $118.50 = $197.50
- Total estimate = $364.00 + $197.50 = $561.50
Quick checks you can do: If you change the trip to 1 day, lodging becomes $0.00 (no overnight stay) and M&IE becomes 0.75 × M for that single day. If you extend the trip by one day, lodging increases by one additional night and M&IE adds one additional full day (because the first and last day are still the reduced days).
Sample rates used in this calculator
The table below lists the sample daily rates available in the Location dropdown. Official per diem tables can include seasonal changes, different rates by county or ZIP code, and separate rules for overseas travel. These simplified values are included to keep the example easy to follow and to make the calculator fast.
| Location | Lodging Rate | M&IE Rate |
|---|---|---|
| Washington, DC | $258 | $79 |
| San Diego, CA | $182 | $79 |
| Fort Liberty, NC | $105 | $64 |
Understanding the results
After you calculate, the results box shows a summary table with five lines: destination, trip length, authorized lodging reimbursement, meals & incidental expenses, and the total estimated per diem. The lodging line is calculated as the nightly lodging rate multiplied by the number of nights. The M&IE line is calculated as a combination of reduced travel days and full days in the middle.
If you are using this estimate for budgeting, it can be helpful to separate the total into “fixed” and “variable” parts. Lodging is often the largest variable because it changes by location and because actual reimbursement may be limited to the lesser of your actual cost or the authorized cap. M&IE is typically more predictable because it is a flat daily amount, but it can still be reduced when meals are provided or when special rules apply.
Common planning scenarios (what to watch for)
Real travel can be messy. The simplified model here is still useful, but it helps to understand where your actual reimbursement may differ. The notes below are written to be practical: they explain why your voucher total might not match a quick estimate and what inputs you would need to refine it.
1) Same-day travel (no overnight stay)
If your trip is a single day, the calculator assumes there are zero lodging nights and applies the 75% M&IE travel-day rule once. Some organizations have additional thresholds (for example, minimum hours in travel status) that determine whether M&IE is payable. This calculator does not model hour-based eligibility; it assumes the day counts.
2) Provided meals or conference meals
Many TDY trips include meals provided by a conference, a host organization, or a dining facility. In those cases, your M&IE may be reduced by a percentage or by meal components (breakfast/lunch/dinner). This calculator does not apply meal deductions; it assumes you are responsible for all meals. If you know meals will be provided, treat the M&IE estimate as an upper bound.
3) Multiple locations or changing rates
If you lodge in different places on different nights, per diem can change by day based on where you are located at midnight (or based on your organization’s rules). Official tables may also change seasonally. This calculator uses one destination and one set of sample rates for the entire trip, so it will not capture day-by-day changes.
4) Actual lodging cost versus the cap
Lodging reimbursement is often limited to the lesser of your actual lodging cost or the authorized lodging cap. The calculator multiplies the cap by the number of nights, which is useful for planning a maximum. If you find lodging below the cap, your actual reimbursement may be lower than the estimate. If you exceed the cap, you may need approval for actual expense or you may be limited to the cap.
5) Travel days and partial days
The 75% rule is a common approach for the first and last day, but policies can vary. Some organizations use different percentages, different rules for local travel, or different rules for certain types of training. This calculator applies a consistent 75% on up to two travel days and full M&IE for the days in between.
Limitations and important notes
This calculator is a planning tool and intentionally simplifies several real-world TDY rules. Use it to understand the mechanics of per diem, then confirm your actual entitlement using your orders and the official rate tables.
- Single destination only: If your itinerary includes multiple locations, per diem can change by day based on where you lodge.
- No provided-meal deductions: Conferences, government dining facilities, or hosted events may require reducing M&IE.
- No actual-expense comparison: Lodging reimbursement is typically limited to the lesser of actual cost or the authorized cap.
- No special cases: This page does not model training reductions, PCS entitlements, government quarters rules, or overseas-specific adjustments.
- Sample rates only: Rates here are not live and may not match current GSA/DoD/State Department values.
If you need an official figure for reimbursement, consult your travel system and the published per diem tables for the correct dates and locations. Still, for quick comparisons and budgeting, this simplified estimate is often enough to answer practical questions like: “How much does one extra day add?” or “How much of the total is lodging versus M&IE?”
Practical checklist before you travel
If you are using this calculator as part of trip planning, the checklist below can help you turn an estimate into a more accurate expectation. It is not a policy document; it is a set of questions that commonly affect the final voucher amount.
- Do your orders specify a particular per diem rate, a reduced rate, or a special authorization (such as actual expense)?
- Will you stay in government quarters or a contracted lodging facility, and does that change your lodging reimbursement?
- Are any meals provided (conference registration, hosted meals, dining facility availability), and will that require M&IE deductions?
- Are you traveling to more than one location, or will you have a night in transit with a different rate?
- Are there seasonal rate changes for your destination during your travel dates?
- Do you have receipts and documentation requirements for lodging and other expenses?
Even if you cannot answer every question in advance, the calculator’s breakdown can still be useful: it shows the maximum lodging portion implied by the cap and the approximate M&IE portion implied by the 75% travel-day rule.