TDY Per Diem 101: How Temporary-Duty Travel Allowances Work
What Per Diem Actually Covers
When you go on temporary duty (TDY), the government reimburses your away-from-home living costs through a daily allowance called per diem. Instead of collecting a receipt for every sandwich and parking meter, per diem sets standard daily amounts tied to where you're staying. It is meant to cover the ordinary, unavoidable costs of being on the road, not to turn a profit and not to leave you out of pocket for the basics.
Per diem is built from two distinct parts: a lodging allowance and a meals and incidental expenses allowance, usually shortened to M&IE. Lodging covers the cost of your room. M&IE covers your meals plus a small, defined set of incidental costs such as tips for hotel and baggage staff. It's worth understanding these as two separate buckets, because they follow very different rules once you file your voucher.
Per diem is location-based. The rate for a night in a high-cost city is different from a rate in a rural county, because the underlying prices are different. Your authorized rate is generally driven by the location where you spend the night on official duty. Big-ticket travel costs like your airfare, rental car, or mileage are reimbursed separately as transportation expenses and are not part of your per diem.
Lodging and M&IE Work Very Differently
The lodging portion is a ceiling, not a flat payment. You are reimbursed for what you actually pay for your room, up to the maximum lodging rate for that location, and you're expected to keep the receipt. Spend less than the cap and you're reimbursed the lower amount; there's generally no reward for coming in under it. Go over the cap and the excess typically isn't reimbursed unless your travel office has authorized an exception in advance, so it pays to check before you book something above the limit.
M&IE works the opposite way. It's paid as a flat daily amount for the location, and you generally don't submit meal receipts to claim it. Because it's a set figure rather than a reimbursement of actual costs, spending less on food doesn't reduce what you receive, and spending more doesn't increase it. The one adjustment nearly every traveler encounters is the first and last day of travel: on those days you're paid 75% of the full daily M&IE rate, since you're only partially away from home. Other reductions can apply when meals are furnished to you at government expense, such as meals provided at a conference or included in a registration fee, but exactly how those deductions are handled can vary by situation, so confirm the treatment in your travel system or with your travel office.
The practical upshot: budget for lodging against the cap and keep your receipts, but treat M&IE as a fixed daily amount you don't have to document meal-by-meal. Knowing which bucket a cost falls into is the difference between a clean voucher and one that bounces back for corrections.
CONUS vs. OCONUS: Why Location Category Matters
Where your TDY takes you determines not just the dollar amount but which rulebook applies. CONUS means the continental United States, the 48 contiguous states plus the District of Columbia. OCONUS means outside the continental United States, and it splits into two meaningfully different groups: non-foreign OCONUS and foreign OCONUS.
Non-foreign OCONUS covers U.S. locations that sit outside the lower 48, including Hawaii, Alaska, and the U.S. territories and possessions. These are still U.S. destinations, but they fall under a different rate authority than the mainland. Foreign OCONUS is everywhere abroad, and those locations follow yet another set of rates that can update frequently and may include country-specific considerations.
The category matters because it tells you where to look up your authorized rate and whose rules govern the trip. Rather than assume the mainland approach carries over, verify the location category and the current rate for your specific destination before you travel, especially for OCONUS trips where the numbers and the fine print change more often.
Who Sets the Rates
Three different authorities publish per diem rates, and which one applies depends on your destination category. For CONUS locations, the General Services Administration (GSA) sets the rates. GSA also establishes a standard rate that applies to the many locations that aren't individually listed as higher-cost areas, so a destination without its own specific rate still has a defined amount.
For non-foreign OCONUS locations such as Hawaii, Alaska, and the U.S. territories, rates are set by the Department of Defense through its Defense Travel Management Office (DTMO). For foreign OCONUS locations, the Department of State establishes the rates. This division is why a TDY to Honolulu, a TDY to Denver, and a TDY to Tokyo each pull from a different source even though the per diem concept is identical.
These authorities set the numbers, but the procedures for authorizing and claiming travel live in your governing travel regulations. Non-DoD federal civilian travel is generally governed by the Federal Travel Regulation (FTR), while DoD travelers — both uniformed service members and DoD civilian employees — follow the Joint Travel Regulations (JTR). Individual agencies layer their own guidance on top. Because the specifics of what's authorized, what requires pre-approval, and how exceptions are handled can differ by agency and situation, always confirm the current rate for your location and the rules that apply to you in DTS or with your travel office before you commit to a booking.
Putting It Together Before You Travel
A smooth TDY comes down to a few habits. Confirm your destination's category and pull the current per diem rate for that exact location, since rates change and the wrong figure leads to voucher problems. Book lodging at or under the maximum lodging rate, or get an exception authorized in writing beforehand if you can't. Keep your lodging receipt, and remember you generally don't need meal receipts for the flat M&IE amount.
Plan for the first and last day of travel to pay 75% of the full daily M&IE rate, and account for any meals furnished at government expense that may reduce your M&IE for those days. When anything is unclear, resolve it up front in your travel system or with your travel office rather than guessing on the voucher.
Getting the concepts right — the two buckets, the location categories, and the rate authorities — means the arithmetic is the easy part. That's exactly where a calculator earns its keep: once you know the rules, you just need the current numbers for your location and your travel dates, and Perdiemify handles the math so you can focus on the trip.
This guide is general educational information, not official travel guidance. Per diem rates, entitlements, and procedures change and vary by agency, location, and situation. Always confirm the current rate for your destination and the rules that apply to you in DTS or with your agency travel office before booking or filing a voucher.