Narrative:

While approaching runway xy (shortened on taxiway C) I asked the first officer if that was the point at which the takeoff calculation was made. He said yes. It looked about right; especially since the displaced threshold was in front of us from that intersection. I asked the question because there was a little bit of runway that looked usable to our left. (The runway construction was on the approach end of the runway to our left.)takeoff was uneventful; however; it did seem that we rotated and lifted off a bit closer to the end than I would have expected. That and a query to tower about usable runway by the aircraft behind us made us recheck our work when we got to cruise. It turns out there was no C intersection option available in the computer. We should have queried ATC and back-taxied to the correct position for takeoff.I should have been more vigilant in determining the correct takeoff point on the runway. Although it was a bit deceiving due to the displaced threshold being to our right (towards departure end of runway) I should have called ATC to verify. There should be a dispatch note on the release to advise crews to back-taxi from C on that runway. It's an easy mistake to make but one with potentially grave consequences. We were lucky that we were not heavier.

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Original NASA ASRS Text

Title: B737NG flight crew reported mistakenly departing from a taxiway intersection even though the takeoff calculations were for using the entire runway.

Narrative: While approaching Runway XY (shortened on Taxiway C) I asked the First Officer if that was the point at which the takeoff calculation was made. He said yes. It looked about right; especially since the displaced threshold was in front of us from that intersection. I asked the question because there was a little bit of runway that looked usable to our left. (The runway construction was on the approach end of the runway to our left.)Takeoff was uneventful; however; it did seem that we rotated and lifted off a bit closer to the end than I would have expected. That and a query to Tower about usable runway by the aircraft behind us made us recheck our work when we got to cruise. It turns out there was no C intersection option available in the computer. We should have queried ATC and back-taxied to the correct position for takeoff.I should have been more vigilant in determining the correct takeoff point on the runway. Although it was a bit deceiving due to the displaced threshold being to our right (towards departure end of runway) I should have called ATC to verify. There should be a Dispatch note on the Release to advise Crews to back-taxi from C on that runway. It's an easy mistake to make but one with potentially grave consequences. We were lucky that we were not heavier.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.