Flight number record
DL 821
Delta Air Lines · Atlanta to Boston
- Route
- ATL → BOS
- Departs
- 17:55
- Flights
- 325
- Period
- July 2025 to June 2026
Historical statistics for July 2025 to June 2026, not live status. For today’s DL 821, check with Delta Air Lines.
DL 821 sits in the middle of the pack with a C. About seven flights in ten arrive on time by the DOT's definition, 9 points worse than the national rate of 77.6%. The late ones land about an hour behind schedule. That rests on 325 flights, with a median arrival 2 minutes late.
The national on-time rate over the same period is 77.6%, so DL 821 runs 9 points behind the country as a whole. Across a thousand flights that is about 92 extra late arrivals.
Key figures
On time
68.4%
Arrivals under 15 minutes late, of 320 completed flights
Avg delay
60 min
Across delayed arrivals only, not all flights
Cancelled
1.54%
National rate is 1.81%
1 hour+ late
11.2%
National rate is 8.1%
These figures cover 325 flights between July 2025 and June 2026. Beyond the headline rate, 11.2% of arrivals land a full hour or more behind schedule. That is the number that wrecks a connection, and it is not visible in an on-time percentage alone.
Schedule
When DL 821 flies
DL 821 flies ATL to BOS without a single fixed slot. Its most common scheduled departure was 17:55, used on 55 of 325 flights, with 18:25 (54 flights) and 18:15 (46 flights) next. Carriers retime flights through the season, so the rates here blend slightly different positions in the aircraft's day.
- Departs 17:5555 flights16.9%
- Departs 18:2554 flights16.6%
- Departs 18:1546 flights14.2%
Scale runs 0% to 100%. Rows with fewer than 30 flights show a dash.
Share of scheduled departures at each local time. Times used on fewer than one flight in ten are not listed.
A 17:55 departure sits in the 5pm–8pm window. Across every US flight scheduled in that window the on-time rate is 68.2%, and DL 821 runs level with that at 68.4%. By that time of day most aircraft have flown several legs already, and whatever went wrong earlier is still in the schedule.
Against the route
DL 821 next to ATL to BOS
Every carrier on ATL to BOS combined arrived on time 74.5% of the time across 4,935 flights, so DL 821 is 6 points behind it. Delta flew 3,666 flights on the same city pair under all its flight numbers, including this one, and those ran 76.2% on time. Across its whole network Delta Air Lines managed 81.6%, which this flight trails by 13 points.
| Compared with | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| DL 821thin sample | 325 | 68.4% | 60 min | 1.54% | C |
| All Delta Air Lines on ATL–BOS | 3,666 | 76.2% | 59 min | 1.72% | B |
| All carriers on ATL–BOS | 4,935 | 74.5% | 63 min | 1.95% | C |
| Delta Air Lines, whole network | 1,038,570 | 81.6% | 71 min | 1.62% | B |
Across the year
DL 821 quarter by quarter
DL 821 did not run the same all year. Its best quarter was Jan–Mar 2026 at 76.7% on time and its weakest Jul–Sep 2025 at 53.7%, a swing of 23 points on roughly 81 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.
- Jul–Sep 202554 flights53.7%
- Oct–Dec 202591 flights67.4%
- Jan–Mar 202689 flights76.7%
- Apr–Jun 202691 flights70.3%
Scale runs 45% to 80%. Rows with fewer than 30 flights show a dash.
By weekday, DL 821 did best on Tuesday at 76.2% on time and worst on Sunday at 57.4%. Each day rests on about 46 flights, so a gap of 19 points is worth noticing but could still narrow in another year.
Delay causes
Why DL 821 runs late
One category carries most of the weight here. National Airspace System accounts for 49% of the 6,093 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.
- 49%National Airspace System
- 26%Carrier
- 17%Late-arriving aircraft
- 9%Extreme weather
49% of delay minutes are charged to the National Airspace System: air traffic volume, runway configuration, ground stops. No carrier can price its way out of that, which is why airport choice moves the number here more than carrier choice does. That is 2,965 minutes out of 6,093, against a national NAS share of 21%. The split is drawn from 6,093 delay minutes reported on DL 821.
Cancellations sit at 1.54% against a national 1.81%, which is ordinary. Diversions add another 0.00%.
Connections and routes
What a delay on this flight costs
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 11.2% of arrivals land a full hour or more behind schedule, against 8.1% nationally. A 45-minute layover does not survive that, and no amount of walking quickly changes it. Ninety minutes is the honest minimum here, and two hours if the onward flight is on a separate ticket.
The 946 miles between these two airports put this in the middle of the US network, around two and a half to three hours of block time. That is enough cruise to absorb a ten-minute late pushback and not enough to absorb thirty. It is also long enough that the aircraft usually flies three or four legs a day rather than six, which slows how fast delay compounds.
Every one of the 325 flights under DL 821 in the data flew ATL to BOS. The number was not reused on any other city pair, which makes this a clean record of one route.
One caution: this is a thin sample. 325 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
What to do with this
Booking DL 821
For a booking, DL 821 is one of the weaker ways to fly ATL to BOS: 6 points below the route as a whole. If the schedule allows, compare it with the other departures listed on the route page before settling on this one.
Related
Where to go next
- ATL to BOS, every carrier and departure
- BOS to ATL, the return direction
- Delta Air Lines 855 on the same route88.1%
- Delta Air Lines 865 on the same route79.8%
- Delta Air Lines 726 on the same route69.5%
- Delta Air Lines 350 on the same route75.5%
- Delta Air Lines 300 on the same route68.5%
- Delta Air Lines 987 on the same route89.8%
- Hartsfield-Jackson Atlanta International (ATL) delay statistics
- Boston Logan International (BOS) delay statistics
- Delta Air Lines on-time record
- Every Delta Air Lines flight number with a record
- The most and least reliable flight numbers in the US
- How often a connection fails at the major US hubs
A flight number here means Delta Air Lines’s own number as it reports to the DOT. How the figures are computed is on the methodology page.
Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.