Birkenstock shoe size, help!?

I have had my pair of birkenstocks for years and they are absolutly wonderful!!! They are finally falling apart and I want to order a new pair (exactly like the old pair!) on line to save money. The only problem is I'm not sure what size mine are. Beore the birkenstock trademark on the sole it says 36 L5, but then below made in Germany it says 45. I looked up Europe to US shoe sizes and the 36 would be a size 5...but the size 45 would be a 12. I'm a size 7 so neither number makes any sense. How do I know what size my original sandles are..because it was perfect!!