Every figure in Group Trip Cost is published data, a model fitted to published data, or a number you typed — and the calculator marks which is which. This page explains the models, what they were checked against, and where they are weakest.
When six people leave from four cities, a single price per person describes nobody’s trip. One party drives four hours; another buys three seats. The calculator prices each party’s own journey, adds the lodging and daily spending the group shares, and gives every destination one total for the whole group. Destinations are then ranked on that total against travel time, and each party’s own share is shown separately, because it is rarely equal.
The General Services Administration publishes the most a federal employee may claim for a hotel night, county by county and month by month. For FY2027 it rates 295 localities, and everywhere else gets the standard $113 a night. That is a cap on a hotel room, not the price of a house for a group, so it is used as an index rather than a price:
a night for a place sleeping four = per-diem rate × K
K is measured rather than assumed. In six markets, whole-home listings were priced for the last week of July 2026 — the median of three to six listings at each point, all fees included — and divided by the per-diem rate in force at the time:
| Market | Kind | Per-diemwhen measured | Real listingsleeps four | K |
|---|---|---|---|---|
| Charleston, SC | city | $244 | $290 | 1.19 |
| Destin, FL | beach | $260 | $326 | 1.25 |
| Traverse City, MI | lake | $235 | $387 | 1.65 |
| Asheville, NC | mountain | $120 | $244 | 2.03 |
| San Diego, CA | beach | $237 | $554 | 2.34 |
| Vail, CO | mountain | $201 | $554 | 2.76 |
K does not settle by kind of trip. San Diego, CA and Destin, FL are both beach towns, and their K values are 1.86× apart — as wide as the gaps between kinds. The likeliest reason is how scarce whole-home rentals are compared with hotels: a place with plenty of hotels and tightly regulated short-term rentals gets a modest per-diem rate and expensive houses. No trip category captures that, which is why K is measured market by market wherever it can be.
Measured markets use their own K. Everywhere else uses the median for its kind of trip (cities 1.19, lakes 1.65, beaches 1.80 and mountains 2.39), and national-park gateways borrow the mountain figure. Those are realistic to perhaps half either way: a starting point to type over, not a quote. A measured market moves with its own county's change: Destin was measured at $326 and prices at $344 on FY2027 rates.
GSA rates many places by season, so each destination carries a month-by-month curve relative to July. Breckenridge runs $709 a night in January against $393 in April; Destin runs $344 a night in June against $142 in January, for the same property. If you set dates, the calculator prices the month you travel; without them it uses July.
The curve follows federal travel, which matches holiday demand in most places and runs the other way in a few. Jackson Hole reads cheapest in January, because government travel there peaks in summer, not in ski season. Where that happens the destination page says so. GSA rates one of the 40 at one price all year. The calculator lets you turn the season’s weight down, or off.
a night for the group = sleeps-four rate × (1 + 2.44 × ln(people ÷ 4)), above four people
Calibrated on Destin with real listings for four, eight and twelve guests: $326, $878 and $1,194 a night. Fitted on the first two, the curve predicts the third within 0.6%. A straight charge per extra person, fitted the same way, misses it by 20%.
Price follows bedrooms and position, not headcount. Going from four people to eight usually means a different class of property, and that step is the expensive one; filling the bigger house costs much less. Per person it is lowest for four, highest for eight, and falls slowly after that. At Destin’s current rate:
| Group | A nightwhole place | Per persona night |
|---|---|---|
| 4 people | $344 | $86 |
| 6 people | $684 | $114 |
| 8 people | $926 | $116 |
| 10 people | $1,113 | $111 |
| 12 people | $1,266 | $106 |
Food comes from the same schedule’s meals-and-incidentals rate for each location, less the $5 incidental slice, so it varies by place. The standard rate works out at $63 a day ($16 breakfast, $19 lunch, $28 dinner); the dearest locations reach $87. Children are counted at a share of an adult’s daily spending, 50% unless you change it. No dataset settles that figure, so it is a visible setting rather than a hidden assumption. Activities and local transport have no published source at all, and are left for you to fill in.
The Department of Transportation’s Domestic Airfare Consumer Report publishes the average fare between busy airport pairs each quarter. Where it covers a route the calculator uses it directly: 398 pairs relevant to this library, from 2026 Q1. It covers the busiest city markets, so leisure airports are often missing — 31 of the library’s 57 airports have no published pair at all.
Everywhere else, a round trip is estimated from distance with a line fitted to 2,073 published airport-pair fares:
round trip = $369 + $0.090 × one-way miles, and at least $200
Distance explains surprisingly little: R² is 0.24 on the average fare and 0.13 on the lowest-fare carrier, which is the column the calculator prices from. Most of a ticket is fixed cost:
| Distance | Airport pairs | Median fareeach way | Per mile |
|---|---|---|---|
| 0–400 miles | 198 | $232 | $0.78 |
| 800–1,200 miles | 513 | $249 | $0.25 |
| 2,600+ miles | 43 | $335 | $0.13 |
Published averages include last-minute business travel, so a family booking months ahead may well pay less. Every estimated fare is labelled as one, and typing a real fare over it makes it yours.
A plane seat is priced per person; a car is priced once, whoever is in it. For a family within a day’s drive that can flip the answer. Driving is costed at the IRS standard mileage rate, 76 cents a mile, which covers running the car rather than only its fuel. Road distance and average speed both depend on the length of the trip — short ones wind through towns, long ones run on interstates — and the model was checked against eight real routes, landing within 6%. A new option defaults to driving up to 600 miles and flying beyond. Drives over ten hours are shown in days, and the nights spent on the road are not costed, so a very long drive looks cheaper here than it will be.
19 of the 59 places in the library have no price yet. 17 are outside the continental US, where federal rates come from different schedules that have not been added; two are places GSA does not rate separately. Giving those the national standard rate would make them look like the cheapest places in the library, which is exactly backwards, so they stay blank until priced properly. A blank figure stops an option from being ranked: an empty field never quietly counts as zero.
Options are ranked on a blend of total cost and travel time — 60% cost and 40% time unless you move the slider. A contingency buffer adds a planning range to every total; because it scales every option equally, it never changes the order. How well a place suits children, adults or limited mobility is shown alongside the ranking and never folded into it.
GSA publishes a new year of rates each August for the year starting 1 October; the library is on FY2027. DOT publishes each quarter roughly six months after it ends; the fares are from 2026 Q1. Library figures carry the date they were recorded, and the calculator flags any more than 120 days old.
A correction is the most useful thing anyone can send: which place, what you actually paid, for how many people and when. Contact details are on the about page.
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