"Blocked by [the cause]" names the actual obstacle. "Blocked on [what you're waiting for]" names the pending item. Both are natural, idiomatic internal-project register with different emphasis -- not one right and one wrong.
Not a grammar error -- a focus shift
The risk with this pair isn't producing an ungrammatical sentence. It's treating the two as interchangeable without noticing the shift in focus each one creates.
"I'm blocked by a permissions issue -- IT needs to grant me admin access first."
"Blocked by" points the sentence at what's causing the stoppage -- the permissions issue itself is doing the grammatical work.
"I'm blocked on the API team's response before I can finish the integration."
"Blocked on" points at what the speaker is waiting on to clear -- the API team's response is the pending item, not necessarily framed as "the cause."
Both forms can appear in the same situation
The same real-world situation can often be described accurately with either form, depending on what you want to foreground:
"She's blocked on the vendor's quote, but the actual cause is their slow procurement process."
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Start learning for free →Notice that this sentence uses both forms deliberately -- "blocked on the vendor's quote" names the pending item, and "the actual cause is their slow procurement process" separately names the underlying reason. That's not redundant; it's two different, useful pieces of information in one sentence.
Picking the one that matches your emphasis
A rough rule of thumb: reach for "blocked by" when you want to name the mechanism or root cause, and reach for "blocked on" when you want to name what you're specifically waiting to receive. Neither choice is wrong when both are true at once -- pick whichever one your listener needs first.
The mistake to avoid
Mistake: assuming one form must be wrong because it differs from the other. Both "blocked on" and "blocked by" are correct, idiomatic internal-project register with different emphasis. Don't correct a colleague's use of one in favor of the other -- unless the sentence genuinely doesn't match what they mean, in which case the fix is about accuracy, not about the preposition itself.
Why this pair is worth learning deliberately
Most near-synonym pairs in English have a real right-or-wrong answer once you know the rule. This one doesn't, and that's exactly what makes it worth calling out on its own -- a learner who's used to hunting for "the correct form" can spend real effort trying to figure out which of "blocked on" and "blocked by" is right, when the actual skill is recognizing that both are, and choosing based on what you want your sentence to foreground.
Once that reframe clicks -- there's no single correct answer to find, only a better or worse fit for what you want to emphasize -- the pair stops feeling like a trap and starts feeling like a small, useful bit of flexibility in how precisely you can phrase a status update.
Practice scenarios
Practice using blocker in situations like:
- choosing "blocked on" to name what you're waiting for
- choosing "blocked by" to name the actual cause
- using both in the same sentence when the pending item and the root cause are worth naming separately
Useful practice phrases:
- "I'm blocked by [cause] -- [next step]."
- "I'm blocked on [pending item] before I can [next step]."
- "[Person/team] is blocked on [item], but the actual cause is [root cause]."
Neither form is the "correct" one and the other a mistake.
Pick the one that puts what your listener needs to know first.
Lyra Practice helps advanced non-native English professionals learn the nuance of high-value workplace expressions and practice using them in realistic scenarios, so their English sounds natural, precise, and senior at work. Try Lyra Practice.