Flight number record

DL 321

Delta Air Lines · Boston to Los Angeles

Route: BOS → LAX
Departs: 20:35
Flights: 284
Period: September 2025 to August 2026

Historical statistics for September 2025 to August 2026, not live status. For today’s DL 321, check with Delta Air Lines.

DL 321 draws a C. About one flight in three arrives at least 15 minutes late, and the typical late arrival is an hour and a half behind schedule. Build slack into anything you connect to. That rests on 284 flights, with a median arrival 2 minutes late.

The national on-time rate over the same period is 77.3%, so DL 321 runs 12 points behind the country as a whole. Across a thousand flights that is about 115 extra late arrivals.

On time

65.8%

Arrivals under 15 minutes late, of 275 completed flights

Avg delay

89 min

Across delayed arrivals only, not all flights

Cancelled

2.82%

National rate is 1.89%

1 hour+ late

14.2%

National rate is 8.3%

These figures cover 284 flights between September 2025 and August 2026. Beyond the headline rate, 14.2% of arrivals land a full hour or more behind schedule, against 8.3% nationally.

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Schedule

When DL 321 flies

DL 321 flies BOS to LAX without a single fixed slot. Its most common scheduled departure was 20:35, used on 81 of 284 flights, with 20:30 (49 flights) and 17:30 (38 flights) next.

Departs 20:3581 flights28.5%
Departs 20:3049 flights17.3%
Departs 17:3038 flights13.4%

Scale runs 0% to 100%.

Share of scheduled departures at each local time. Times used on fewer than one flight in ten are not listed.

A 20:35 departure sits in the 8pm–5am window. Across every US flight scheduled in that window the on-time rate is 70.1%, and DL 321 runs 4 points worse at 65.8%. By that time of day most aircraft have flown several legs already, and whatever went wrong earlier is still in the schedule.

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Against the route

DL 321 next to BOS to LAX

Every carrier on BOS to LAX combined arrived on time 73.7% of the time across 4,794 flights, so DL 321 is 8 points behind it. Delta flew 1,238 flights on the same city pair under all its flight numbers, including this one, and those ran 73.9% on time. Across its whole network Delta Air Lines managed 81.1%, which this flight trails by 15 points.

DL 321 against its route and carrier
Compared with	Flights	On time	Avg delay	Cancelled	Grade
DL 321thin sample	284	65.8%	89 min	2.82%	C
All Delta Air Lines on BOS–LAX	1,238	73.9%	74 min	2.18%	C
All carriers on BOS–LAX	4,794	73.7%	79 min	1.81%	C
Delta Air Lines, whole network	1,039,517	81.1%	71 min	1.74%	B

Across the year

DL 321 quarter by quarter

DL 321 did not run the same all year. Its best quarter was Jan–Mar 2026 at 77.6% on time and its weakest Jul–Sep 2026 at 53.5%, a swing of 24 points on roughly 71 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 202517 flights—
Oct–Dec 202553 flights58.0%
Jan–Mar 202689 flights77.6%
Apr–Jun 202680 flights70.0%
Jul–Sep 202645 flights53.5%

Scale runs 45% to 85%. Rows with fewer than 30 flights show a dash.

By weekday, DL 321 did best on Tuesday at 72.5% on time and worst on Monday at 57.1%. Each day rests on about 41 flights, so a gap of 15 points is worth noticing but could still narrow in another year.

Arrival by arrival

How late DL 321 runs when it is late

The 94 late arrivals split 27 under half an hour, 28 between 30 and 60 minutes, and 39 an hour or more.

15+ min early53 flights19.3%
Within 15 min128 flights46.5%
15–29 min late27 flights9.8%
30–59 min late28 flights10.2%
1–2 hours late21 flights7.6%
2+ hours late18 flights6.5%

Scale runs 0% to 100%.

Share of 275 completed flights by arrival against schedule.

26 of 96 late departures made up the time and arrived on time, 27% of them. In the other direction, 24 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at LAX.

The worst days

Worst days for DL 321

The latest arrival of the period was on 26 January 2026, 16 hours 28 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The same cause led on four of the five worst days.

The latest arrivals of DL 321 in the period
Date	Arrived late by	Largest filed cause
26 January 2026	16 hours 28 minutes	Carrier
11 March 2026	14 hours 5 minutes	Carrier
13 October 2025	5 hours 31 minutes	Carrier
14 October 2025	5 hours 14 minutes	Late-arriving aircraft
27 March 2026	4 hours 3 minutes	Carrier

Delay causes

Why DL 321 runs late

Carrier accounts for 45% of the 8,365 delay minutes carriers reported here, and the remaining four split what is left.

Carrier
45%
Late-arriving aircraft
44%
National Airspace System
11%
Extreme weather
0%

44% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 8,365 reported delay minutes here, that single category outweighs weather by 44 to one. The split is drawn from 8,365 delay minutes reported on DL 321.

Cancellations run at 2.82%, against 1.89% nationally. That is roughly one flight in 40 cancelled outright. A cancelled flight is invisible in every on-time figure on this page, so the punctuality numbers above describe only the flights that actually operated.

Connections and routes

What a delay on this flight costs

14.2% of arrivals land a full hour or more behind schedule, against 8.3% nationally. Ninety minutes is the honest minimum for a connection here, and two hours if the onward flight is on a separate ticket.

At 2,611 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.

Every one of the 284 flights under DL 321 in the data flew BOS to LAX. 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. 284 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 321

For a booking, the cancellation rate is the figure to plan around on DL 321: 2.82% of its 284 scheduled flights did not operate. A late arrival costs an hour and a half, but a cancellation costs the day, so leave a later option open.

Where to go next

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, September 2025 to August 2026. Last updated 22 October 2026; recomputed monthly. How this data is collected.

Key figures