Lunitidal intervals measure the difference between moon up -- when it's directly over your longitude -- and the high tide immediately following. Theoretically, these times are the same, but in reality, they're usually slightly different due to the friction on the ocean floor changing the tide in many locations. Thankfully, calculating the lunitidal interval in your area, or any area, is fairly straightforward.
Find the moon-up time on a lunar calculator by selecting your location. You can choose today's date, or a specific date. You'll need the same date for use later, so write it down. You'll need the moon-up time in a 24-hour format; for example, 8 a.m. is 08:00, and 8 p.m. is 20:00.
Find the high tide time for the same date. For example, on a website such as Saltwater Tides, select your state, then city or region, and enter the date. If the dates are limited, select one from the website, then go back to the first step and find the moon up for that date. This information is also frequently available from newspapers located in a coastal area.
Calculate the lunitidal interval using the determined times. Subtract the moon up time from the high tide time, which gives you the lunitidal interval. For example, if moon up is at 16:05:05, and high tide time is at 16:10:10, the lunitidal interval is 00:05:05, or five minutes and five seconds.