# Time measurement (OpenMP)
**Time measurement** in OpenMP is done with `omp_get_wtime()`, which returns elapsed wall-clock time in seconds as a `double`. Calling it before and after a parallel region gives the actual time the user waited, which is what matters for speedup measurement. CPU time is not useful here: it sums across all threads and grows with thread count rather than shrinking.

```c
double t0 = omp_get_wtime();

#pragma omp parallel for
for (int i = 0; i < N; i++)
    a[i] = foo(i);

printf("%.4f s\n", omp_get_wtime() - t0);
```

Speedup is the serial wall time divided by the parallel wall time. To get a meaningful serial baseline, either run the same binary with `OMP_NUM_THREADS=1` or time the region before adding any pragmas.