Flight number record

DL 321

Delta Air Lines · Boston to Los Angeles

Route: BOS → LAX
Departs: 20:35
Flights: 275
Period: July 2025 to June 2026

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

DL 321 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 an hour and a half behind schedule. That rests on 275 flights, with a median arrival 2 minutes late.

The national on-time rate over the same period is 77.6%, so DL 321 runs 9 points behind the country as a whole. Across a thousand flights that is about 89 extra late arrivals.

On time

68.7%

Arrivals under 15 minutes late, of 268 completed flights

Avg delay

92 min

Across delayed arrivals only, not all flights

Cancelled

2.18%

National rate is 1.81%

1 hour+ late

11.9%

National rate is 8.1%

These figures cover 275 flights between July 2025 and June 2026. Beyond the headline rate, 11.9% 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 321 flies

DL 321 flies BOS to LAX without a single fixed slot. Its most common scheduled departure was 20:35, used on 95 of 275 flights, with 17:30 (38 flights) and 17:59 (36 flights) next. Carriers retime flights through the season, so the rates here blend slightly different positions in the aircraft's day.

Departs 20:3595 flights34.5%
Departs 17:3038 flights13.8%
Departs 17:5936 flights13.1%

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 20:35 departure sits in the 8pm–5am window. Across every US flight scheduled in that window the on-time rate is 70.5%, and DL 321 runs level with that at 68.7%. 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 321 next to BOS to LAX

Every carrier on BOS to LAX combined arrived on time 74.9% of the time across 4,713 flights, so DL 321 is 6 points behind it. Delta flew 1,228 flights on the same city pair under all its flight numbers, including this one, and those ran 75.2% on time. Across its whole network Delta Air Lines managed 81.6%, which this flight trails by 13 points.

DL 321 against its route and carrier
Compared with	Flights	On time	Avg delay	Cancelled	Grade
DL 321thin sample	275	68.7%	92 min	2.18%	C
All Delta Air Lines on BOS–LAX	1,228	75.2%	76 min	1.71%	B
All carriers on BOS–LAX	4,713	74.9%	76 min	1.42%	C
Delta Air Lines, whole network	1,038,570	81.6%	71 min	1.62%	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 Oct–Dec 2025 at 58.0%, a swing of 20 points on roughly 69 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 202553 flights62.3%
Oct–Dec 202553 flights58.0%
Jan–Mar 202689 flights77.6%
Apr–Jun 202680 flights70.0%

Scale runs 50% to 85%. Rows with fewer than 30 flights show a dash.

By weekday, DL 321 did best on Friday at 73.8% on time and worst on Sunday at 60.5%. Each day rests on about 39 flights, so a gap of 13 points is worth noticing but could still narrow in another year.

Delay causes

Why DL 321 runs late

Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 7,725 reported delay minutes.

Carrier
42%
Late-arriving aircraft
41%
National Airspace System
14%
Extreme weather
3%

41% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. That is a cascade, and cascades start small and grow through the day. Across 7,725 reported delay minutes here, that single category outweighs weather by 12 to one. It is the strongest argument for booking the first departure of the morning, when the aircraft has been sitting at the gate overnight with nothing to inherit. The split is drawn from 7,725 delay minutes reported on DL 321.

Cancellations sit at 2.18% against a national 1.81%, which is ordinary. Diversions add another 0.36%.

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. 11.9% 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.

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.

The same number turns up on one other city pair in the data, including DEN to ATL (1), but none flew often enough under DL 321 to report a rate for. Carriers move flight numbers between routes, so the figures on this page apply to BOS to LAX only.

One caution: this is a thin sample. 275 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, DL 321 is one of the weaker ways to fly BOS to LAX: 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.

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, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.

Key figures