The first time we tried to cool down an outdoor pool in 34°C weather, we severely underestimated the amount of ice required. $100 worth of ice disappeared in ONE MINUTE and the temperature only dipped slightly. Sorely disappointing and too late to order more ice last minute!
A few google searches later, we now have a decent ice cube estimation sheet so that other people will not face the same uncertainty next time.

Click the below link to use the online ice bath calculator, where you just have to input:
-Tub dimensions
-Number of ice bags & Weight per bag
-Water's room temperature
*** Measurements are all in metric ***