TDY Per Diem Calculator

Introduction: what per diem actually pays for on a TDY trip

Temporary duty travel can feel simple on the calendar but surprisingly complicated on the voucher. A short trip usually includes at least two moving parts: lodging, which is often reimbursed up to an authorized nightly cap, and meals & incidental expenses (M&IE), which are usually paid at a flat daily rate. Then another rule enters the picture: many travel systems pay only 75% of M&IE on the first and last travel day. That is easy to forget when you are comparing trips, estimating a budget, or deciding whether a nearby hotel still fits inside the cap.

This TDY per diem calculator is designed to make that logic visible. Instead of forcing you to work the numbers on paper, it lets you select a sample destination, enter the number of days away, and see the estimate broken into its main pieces. The result is not an official authorization and it does not replace travel orders or published rate tables, but it is very useful for planning. In one pass, you can see how an extra day changes the total, how much of the estimate comes from lodging, and how much comes from M&IE.

The page also explains the assumptions behind the math so the result is easier to trust and easier to sanity-check. If you are new to TDY travel, the explanation below gives you a practical mental model. If you already understand the basics, you can skip ahead to the form, run a few scenarios, and use the estimate as a quick comparison tool.

How to use this TDY per diem calculator

The calculator follows a deliberately simplified domestic TDY model. It assumes one destination for the whole trip, one nightly lodging cap, one daily M&IE rate, and the familiar reduced-rate rule on the departure and return day. That scope is intentional. Many official systems account for provided meals, seasonal rates, mid-trip location changes, actual lodging cost, and special policy exceptions. Those details matter for reimbursement, but they can hide the core idea when you are only trying to understand the structure of per diem.

Here, the structure is clear: the number of lodging nights is normally one less than the number of travel days, because a three-day trip typically means two nights away. M&IE works differently. The first and last travel day receive 75% of the daily rate, while any middle days receive the full daily amount. Add the two components together and you have a fast planning estimate.

Running a trip scenario through the form

Start with the destination list. Each location in the dropdown includes a sample nightly lodging rate and a sample M&IE rate. Next, enter the total number of calendar days you will be in travel status, including both the departure day and the return day. When you select Calculate Per Diem, the result box builds a short summary table that shows the destination, trip length, lodging reimbursement estimate, M&IE estimate, and combined total.

This is especially helpful when you are comparing options. If the trip length is uncertain, keep the location fixed and test a shorter and longer stay. If the schedule is fixed but the location might change, keep the number of days the same and switch the destination. Because the result stays split between lodging and M&IE, you can tell immediately whether most of the difference comes from hotel caps or from extra daily allowances.

1. Select a destination from the sample list.
2. Enter travel days as the total calendar days away, including departure and return.
3. Calculate the estimate to see the breakdown.
4. Copy Summary if you want to paste the result into a worksheet, email, or travel-planning note.

The per diem formula that turns days into dollars

The calculator uses a compact set of assumptions that match the script on this page. Those assumptions matter because they explain why the total changes the way it does when you add a day or switch locations.

Let L be the nightly lodging cap, M the full daily M&IE rate, and d the total travel days. The page then computes the number of lodging nights, the number of full-rate middle days, the reduced-rate travel days, and the final total. In this simplified setup, the first and last day are the only days reduced to 75%, and any days between them stay at 100% of the M&IE rate.

In that notation, N is lodging nights, F is full-rate M&IE days, R is the M&IE subtotal, and T is the total estimated per diem. The model also assumes that no meals are provided at government expense, that no meal-by-meal deduction applies, and that the same sample rate applies for the whole trip. Those are reasonable assumptions for quick planning, but they are still assumptions.

• Lodging nights are assumed to be travel days minus one.
• M&IE is paid at 75% on the first and last travel day and at 100% on the days in between.
• No provided-meal deductions are taken.
• Rates are sample values for illustration and do not update by season.

Worked example: a three-day San Diego trip

Suppose you travel to San Diego, CA for 3 days. In this calculator, San Diego has a sample lodging cap of $182 per night and an M&IE rate of $79 per day. A three-day trip usually means two hotel nights, so the lodging portion is easy to compute first: 2 × $182 = $364.00.

M&IE takes one extra step. With three travel days, the middle day is the only full-rate day, and the first and last day receive 75% of the daily amount. That becomes $79 for the full day plus 0.75 × $79 × 2 = $118.50 for the two reduced travel days. Add them together and the M&IE subtotal is $197.50. Once you combine lodging and M&IE, the total estimate is $561.50.

• Nights: 3 − 1 = 2
• Lodging total: 2 × $182 = $364.00
• Full M&IE days: 3 − 2 = 1
• M&IE total: $79 + $118.50 = $197.50
• Total estimate: $364.00 + $197.50 = $561.50

This example also shows why a one-day extension can move the estimate in two ways at once. If you add a fourth day, you add one more hotel night and one more full M&IE day. If you shorten the trip to one day, you remove lodging entirely and keep only a single reduced M&IE day.

Sample rates used in this calculator

The destination dropdown uses a short set of sample rates so the calculator stays simple and fast. Official per diem tables can vary by county, ZIP code, season, organization, and travel type. This page does not try to reproduce that full complexity. Instead, it uses a few representative examples so you can practice reading the math and comparing scenarios.

Understanding the result

After you calculate, the result area shows a compact planning summary. The destination and trip length remind you which scenario you entered. The lodging line shows the authorized lodging reimbursement estimate based on the sample nightly cap multiplied by the number of nights. The M&IE line reflects the split between reduced travel days and any full-rate middle days. The final row adds those parts together into a single estimate.

The breakdown is useful because the total alone can hide the reason a trip is expensive. A longer trip may have only a modest M&IE increase but a large lodging increase. A high-cost city may push the total mostly through the hotel cap rather than through meals. When you can see the separate pieces, you can make better planning decisions such as adjusting the schedule, comparing sites, or setting a more realistic travel budget.

Common planning scenarios

Real TDY travel rarely follows the neatest possible example, so it helps to know where a simplified estimate can drift away from the final voucher. The calculator is still valuable in these cases, but you should understand what it is and is not modeling.

Same-day travel

If the trip lasts only one day, the calculator assumes there is no lodging and applies a single 75% M&IE day. Some organizations also have hour-based thresholds that determine whether M&IE is payable at all. Those rules are not modeled here, so the result should be treated as a basic planning estimate rather than a policy determination.

Provided meals

Conferences, host organizations, training programs, and government dining facilities can reduce M&IE when meals are provided. This page does not subtract breakfast, lunch, or dinner components. If you know meals will be furnished, the calculator may overstate the M&IE portion of the trip. That makes the estimate more like an upper planning bound than a final payable amount.

Multiple locations

Some trips involve different lodging locations on different nights. Official per diem may then change by day depending on where you are located under the applicable policy. This calculator uses one destination for the whole trip, so it is best suited to single-location TDY or to rough first-pass budgeting before you break out a more detailed day-by-day analysis.

Actual lodging cost versus the cap

The lodging portion here is based on the authorized nightly cap, not your actual hotel receipt. In practice, reimbursement may be limited to the lesser of actual cost or the authorized cap unless a special actual-expense authority applies. That means the calculator often shows a practical ceiling for lodging rather than a guaranteed reimbursement amount.

Alternative travel-day rules

The 75% first-day and last-day rule is common, which is why it is used here, but organizations can adopt different procedures for special cases. Training reductions, overseas travel, local travel rules, and mission-specific policies can all change the result. The calculator stays intentionally consistent so that the core logic is easy to learn.

Limitations and important notes

This calculator is best understood as a planning and teaching tool. It is specific enough to be useful, but simplified enough to remain fast and readable. That balance is the whole point: you can use it in seconds, understand the output, and then decide whether you need a more formal voucher review.

• Single destination only: it does not model changing per diem locations during the trip.
• No meal deductions: it assumes no conference meals, hosted meals, or dining-facility reductions.
• No actual-expense comparison: it does not compare the cap with your real hotel bill.
• No special policy cases: it does not cover PCS, training reductions, government quarters rules, or overseas-specific methods.
• Sample rates only: it does not connect to live GSA, DoD, or State Department tables.

Even with those limitations, the calculator answers practical questions very well. How much does one more day add? How much of the total is driven by the hotel cap? How much of the allowance is tied to M&IE? For quick planning, that kind of answer is often exactly what you need.

Practical checklist before you travel

Before you rely on any estimate, it is worth checking a few details that commonly change the final number. Think of the list below as a short TDY reality check. It will not replace policy guidance, but it will help you spot the factors most likely to affect reimbursement.

• Do your travel orders specify a reduced rate, an actual-expense authority, or a special local rule?
• Will you stay in government quarters or another facility that affects the lodging portion?
• Are any meals provided by the conference, host unit, or training site?
• Will you spend nights in more than one location or cross a seasonal rate change?
• Do you know whether your expected hotel rate is below, near, or above the authorized cap?
• Do you have the receipts and documentation your organization requires for lodging and other travel expenses?

If several answers are still uncertain, the calculator can still help. Run a conservative scenario, then run a tighter scenario with fewer days or a lower hotel cost assumption. The gap between those estimates gives you a useful planning range while the official details are still being confirmed.

TDY per diem questions travelers ask

Sources: the per diem structure — a nightly lodging ceiling plus a daily M&IE amount, with M&IE paid at 75% on the first and last travel day — follows the U.S. General Services Administration Per Diem Rates and the Department of Defense Joint Travel Regulations (JTR). The built-in city rates are illustrative sample values; always confirm the current official rate for your destination and travel dates.