Stop by your local hardware store and get you one of these:
- A new, high-quality carpenter’s level: A good 4-foot box-beam level often has a machined edge that is very straight.
- The factory edge of 3/4″ MDF: The factory-cut edge of a new sheet of MDF is typically extremely straight.
You also need a cheap set of feeler gauges from any auto parts store.
- Clean the Table: Wipe down the entire surface. Any spatter or debris will ruin the reading.
- Place the Straight Edge: Lay your straight edge (e.g., your level) on its edge diagonally from one corner to the other.
- Check for Light: Get low and shine (A) flashlight from the backside. This will instantly show you any gaps.
- Measure the Gaps: This is the key step.
- Slide the thinnest leaf of your feeler gauge (e.g., 0.002″) into the largest gap.
- If it slides in, try the next size up. Keep stacking leaves or using thicker ones until you find the one that just barely fits with slight friction.
- The thickness of that gauge is your deviation.
- Repeat in All Directions: You must check the table in a “star” pattern:
- Corner-to-corner (Diagonal 1)
- Corner-to-corner (Diagonal 2)
- Along the long edge (Edge 1)
- Along the long edge (Edge 2)
- Along the short edge (Edge 3)
- Along the short edge (Edge 4)
- Through the middle (both ways)
Result: This gives you a real number. You’ll be able to say, “My table has a 0.015-inch dip in the center.”
However, keep in mind that this is also dependent on the flatness of the actual straight edge you have! If you want to be as accurate as possible, perform the exact same measurements with both sides of the straight edge, add your measurements, and divide by 2 to get the average.