August at Costco was a split month. Regular gasoline across the 569 U.S. stations we track finished at $4.20 a gallon — down 3 cents, or 0.7%, from $4.23 on August 1. That near-flat close hides a V: a slide to $4.05 on August 12, then a rebound that gave almost all of it back. Diesel did not play along. Across the 63 warehouses that sell it, diesel rose 25 cents (+4.5%) to $5.79 — the only U.S. grade that ended the month higher. AAA says August was the most expensive August on record, with the national average above $4 every day; Costco's West-heavy mix sat a few cents above that print. It is a different shape from the slide we logged in the July report, and it matches a mixed month across Canada, the UK, and Australia.
| Grade | August 1 | August 31 | Month change |
|---|---|---|---|
| Regular | $4.233 | $4.203 | ▼ $0.03 (0.7%) |
| Premium | $4.929 | $4.874 | ▼ $0.06 (1.1%) |
| Diesel | $5.538 | $5.789 | ▲ $0.25 (4.5%) |
National averages of posted prices at tracked U.S. Costco stations, $/gal. Snapshot of calendar August 2026 — this article is not updated.
A V-shaped month: $4.05 on August 12, then back up
For the first twelve days the national average did nothing but fall, grinding from $4.23 on August 1 to a low of $4.05 on August 12. Then the direction flipped. Regular climbed back through $4.20 by August 22 and spent the last week oscillating in a tight band around that level — so the month closed almost where it opened, 3 cents cheaper, after a 19-cent round trip. Diesel followed the same V on the way down, then overshot: after a $5.44 trough on August 13 it kept rising through month-end to a high of $5.79.
Hawaii and the Midwest fell; the northern Rockies rose
The regional story split. Hawaii's Costco average fell 25 cents a gallon (−5.0%), the biggest state drop, with Kona cutting regular by 54 cents — the largest station move in the country. Maryland, Michigan, Indiana, and Ohio each gave back 13 to 18 cents. Indiana is still the cheapest state at $3.20, and Avon, Indiana, at $3.09, is the cheapest Costco pump we track. The other direction belonged to the northern Rockies and the Northwest: Idaho rose 22 cents (+5.4%), led by Meridian's 37-cent jump, with Montana, Washington, Iowa, and Colorado each up more than 10 cents. Of 569 stations, 326 finished cheaper, 194 dearer, and 49 were unchanged.
Biggest fallers
Biggest risers
| State | Avg now | Change |
|---|---|---|
| Idaho | $4.353 | ▲ $0.22 (5.4%) |
| Montana | $4.251 | ▲ $0.11 (2.8%) |
| Washington | $4.872 | ▲ $0.11 (2.4%) |
| Iowa | $3.757 | ▲ $0.11 (2.9%) |
| Colorado | $3.964 | ▲ $0.10 (2.7%) |
Biggest August movers by state-average price of regular at Costco, August 1 – 31, 2026.
The cheapest and most expensive states right now
Geography still sets the spread. The gap between the cheapest and priciest state averages is $2.10 a gallon— Indiana's $3.20 against California's $5.30. All five of the priciest states are in the West. Washington, which led the country lower in July, is now a riser and the second-dearest Costco state; Hawaii fell hard and still sits third. At the pump extremes, Avon, Indiana, at $3.09 sits $2.87 below Eureka, California, at $5.96.
Cheapest states (regular)
Most expensive states (regular)
| State | Costco avg |
|---|---|
| California | $5.299 |
| Washington | $4.872 |
| Hawaii | $4.736 |
| Alaska | $4.549 |
| Nevada | $4.502 |
State-average price of regular across tracked Costco stations, August 31, 2026. Puerto Rico is excluded — its pumps price in dollars per liter.
Why regular barely moved — and why diesel climbed
The war premium never fully left the pump. Crude spent August in the $80s, with the Strait of Hormuz still the swing variable, and AAA's national average stayed above $4 every day of the month for the first time — on track, then confirmed, as the most expensive August on record, beating 2022. By August 31 AAA's all-station average was about $4.08. Costco's $4.20 national print sits above that because more than a quarter of the warehouses we track are in California and Washington, the country's most expensive fuel states.
Regular's V is that $4 floor asserting itself: a mid-month dip as wholesale eased, then a grind back as refining stayed tight and summer demand held. Diesel is the clearer tell. Distillate cracks stayed elevated while gasoline did not, so the 63 Costco diesel pumps we track rose even as regular oscillated — a 25-cent climb that left diesel at its August high on the last day of the month.
A caution before you read the 0.7% as relief
Regular is 3 cents cheaper than August 1 and 19 cents off the mid-month low. That is not a new downtrend. AAA's record August and Costco diesel at $5.79 are the same market: a $4 gasoline floor with distillate still climbing. If Hormuz shipping keeps limping and refining stays tight, the next thirty days start from here, not from the $3.82 July trough.
How Costco compares with the AAA average
Nationally the two averages are no longer on top of each other. Costco's $4.20 versus AAA's $4.08on August 31 is a 12-cent gap the other way from July, when they nearly tied at $3.90. That is geography, not Costco getting more expensive as a retailer: the club's warehouse map is West-heavy, and Washington rising 11 cents this month pulled the simple national average up even as Indiana, Texas, and Oklahoma — where the club discount is widest — stayed in the $3.20s and $3.30s. State by state the club is still cheaper than the all-station average; the national print just no longer disguises it.
Will gas prices fall in September?
Seasonal demand usually eases after Labor Day, but the floor under $4 is a refining-and-Hormuz story, not a driving-season one. The EIA's outlook still has retail gasoline averaging in the high $3s into late 2026 if shipping recovers; the same report flags that renewed disruption would reverse that quickly. Diesel has less seasonal give: it closed August at a high, not a pause.
California remains its own weather system. Even after a 3-cent Costco dip on the month, the state's $5.30 average is the highest we track, and UC Davis economists still put a long-run premium of about $1.21 a gallon on the 2025–26 refinery closures. If you fill up there, the club discount matters more than the national V.
Costco pump prices are a snapshot of calendar August 2026 (August 1–31) and will differ at the pump today. This site is independent and not affiliated with Costco Wholesale Corporation.
