Spinach is often touted as a good source of calcium. Unfortunately, the calcium in spinach is not available for absorption because of a plant substance called oxalates. The oxalates bind the calcium in spinach so calcium is not absorbable. This holds true for raw or cooked spinach.
That’s the bad news.
Nevertheless spinach is a healthy food, you just can’t count calcium from spinach toward your daily intake.
Substitute other green vegetables that are essentially free of oxalates to get calcium. These include kale, bok choy, and broccoli.