How much coolant do I add to top up if warning light comes on

NEVER OPEN THE COOLANT RESERVOIR WHEN THE ENGINE IS HOT!!!
If the coolant level gets too low, the air gets inside the cooling system and there's no proper circulation. Here's how you can fix it yourself: turn the heater on maximum temperature (inside the car), then start adding BMW coolant (it has to be the BMW coolant) to the overflow tank -cooling reservoir. Some models have a bleeder on the upper radiator house, so you have to loose the bleeder. The fluid will push the air out- also there may be a little of coolant coming out, which is O.K..
Next, tighten the bleeder after the coolant reaches the maximum level,close the reservoir and start the engine and wait till it warms up to the normal operating temperature or in other words, when the cooling fan (the fan is just behind the radiator) turns on and off. One more thing, there should be hot air blowing out of the heater inside your car. If it blows cold air, there's still some air in the cooling system and you should see a mechanic.If you don't, you might over-heat the engine.