If you have an 18x33 oval 4 feet deep, your volume would be APROX 15,500 gallons (actually 15,692). But that's assuming your WATER is 4 feet deep. If it's actually 3' 6" deep, you have 13,750gal, approx. (use 14,000 gallons in your estimates) If it's 3' 9", you have 14700 gal, aprox.(use 14,500 or 15,000 gallons in your estimates).