The Key Difference
Both 'by' and 'until' are prepositions of time, but they mean different things. Use 'by' when you want to say something must finish before or at a specific time — it's a deadline. Use 'until' when you want to say something continues up to a specific time, then stops. Think of 'by' as a deadline and 'until' as the end point of an action.
By vs Until: Side by Side
BY: Shows a deadline. The action finishes at or before this time. Example: 'I need to finish the report by Friday.' (Friday is the deadline — it must be done by then or earlier.)
UNTIL: Shows when an action stops. The action continues up to this time. Example: 'I will work on the report until Friday.' (I work, and stop on Friday.)
By vs Until: Quick Comparison
| Category | By | Until |
|---|---|---|
| Meaning | No later than a specific point in time; a deadline by which something must be completed. | Up to a specific point in time; something continues and then stops at that point. |
| Function | Sets a deadline for the completion of an action. The action must happen at or before the stated time. | Marks the end point of a continuing action or state. The action or state lasts up to that moment. |
| Action Type | Used with actions that are completed or finished — a single event or one-time result. | Used with actions or states that are ongoing or continuous over a period of time. |
| Question Answered | "When is the deadline?" / "When must it be done?" | "How long does it continue?" / "When does it stop?" |
| Positive Example | "Please submit the report by Friday." (The report must be submitted no later than Friday.) |
"She worked until midnight." (She kept working and stopped at midnight.) |
| Negative Example | "Don't send the email by tonight." ✗ (Unnatural — by is rarely used this way in negatives.) | "Don't leave until I get back." ✓ (Stay — and only stop staying — when I return.) |
| Question Example | "Can you finish this by noon?" (Asking whether the task can be completed before noon.) |
"Are you staying until the end of the event?" (Asking whether the ongoing presence continues to the end.) |
| Key Signal Words | deadline, due, finish, complete, submit, send, arrive, deliver, hand in | wait, stay, continue, keep, remain, last, hold on, not stop |
| Interchangeable? | Sometimes they appear similar but are not interchangeable. "Finish by 5 pm" means done at any point before 5 pm. "Finish until 5 pm" is unnatural — you cannot use until with a one-time completion event. However, "Wait until 5 pm" is correct; "Wait by 5 pm" is not. | |
| 🔑 Key Difference: Use by when you are talking about a deadline — a point in time by which a single action must be completed (e.g., "done by Monday"). Use until when you are talking about the end point of a continuous action or state — something that keeps going and then stops (e.g., "open until 9 pm"). A simple test: if you can replace the time word with "no later than," use by. If you can replace it with "up to that point," use until. | ||
Examples
You must submit your homework by 3 p.m.
Deadline · A2 Academic
Please finish your breakfast by 8 o'clock.
Deadline · Everyday usage
I will call you by tomorrow evening.
Promise with deadline · Informal
I will wait until you arrive.
Duration · Everyday usage
The shop is open until 9 p.m.
End time · Informational
Don't eat anything until after the doctor's appointment.
Instruction · Medical context
When to use it
Deadlines & Appointments
Use 'by' when talking about when something must be done or will happen at latest.
"The dentist appointment is by 2 p.m." or "Please finish by Friday."
Duration & Waiting
Use 'until' when describing how long an action or state continues.
"I will study until midnight." or "The museum is open until 6 p.m."
Promises & Plans
Use 'by' for promises about when you will complete something.
"I will call you back by 3 p.m." or "I'll send the email by tomorrow."
Signal words
deadline
must finish
completed
by then or earlier
continues
stops at
lasts until
open until
Common Mistakes
✕
Wrong
I will finish this work until 5 p.m.
✓
Correct
I will finish this work by 5 p.m.
Use 'by' for deadlines. 'Until' suggests the work continues past 5 p.m.
✕
Wrong
The office is open by 9 a.m.
✓
Correct
The office is open until 9 p.m.
'By' is a deadline. 'Until' shows when something ends. The office closes at 9 p.m., not opens by that time.
✕
Wrong
I will work until 5 o'clock PM.
✓
Correct
I will work by 5 o'clock PM.
Use 'by' for a deadline or final point in time, and 'until' for the duration of an action continuing up to that point.
KEY TAKEAWAYS
What to Remember
- Use 'by' to show a deadline when something must be finished before or at that time.
- Use 'until' to show when an action or state continues and then stops at that point.
- 'By' focuses on completion; 'until' focuses on duration and the endpoint of an action.
- Don't use 'by' for actions that continue over a period; use 'until' instead.
- 'By' answers "when must this finish?" while 'until' answers "how long does this last?"