A ready-to-use practical for GCSE and A-level Physics: pupils time 20 swings of a 1 m pendulum and calculate g, by hand or with a free calculator. What’s included (one-page PDF): Ages, timing and curriculum links (W = mg; T = 2π√(L/g); required-practical skills) A clear step-by-step method for groups The formula g = 4π² × L × n² ÷ t² with a worked example (≈ 9.82 m/s²) and the range to expect in class Discussion questions on reaction time, measuring length and measurement uncertainty An extension: could your class detect that gravity is weaker at the equator or up a mountain? Safety notes The calculator is free, works in the browser on school laptops and phones, needs no sign-up and has no ads. You only need string, a light weight (a nut, washer or keys), a metre rule and a stopwatch or phone.