What size Reservoir & Pump do I need for my Flood Tray?

So you're building your own hydroponics flood and drain system from parts and pieces but don't know what pump and reservoir are the right size for your flood tray?  It might take a little math, but the calculations are well worth the effort.

You want to make sure your hydroponic reservoir won't be empty while flooding your grow bed so you don't burn out your submersible water pump.  There are two key factors you need to know to begin: the size of your flood tray and how long your timer will be 'on' during feedings.

• Start by measuring your flood tray.  Then use this formula to find out the volume- Length x Width x Height= Volume
• If the height is in inches, convert it to feet.  Do that by dividing the height by 12 (inches in a ft).  So if the height of your flood tray is 7 inches, it would be .58 ft (7/12=.58)
• Now figure out the volume in cubic feet (so a 2' x 4' x .58' flood tray would be 4.64 cubic feet)
• There are 7.48 gallons in 1 cubic foot (so in 4.64 cubic feet there are 34.7 gallons- 4.64 x 7.48= 34.7)  So for a 2' x 4' flood tray to be flooded fully would require a minimum of a 36 gallon reservoir (as you do not want to empty the reservoir completely).  One way to use less water is to fill your flood tray with grow media.  The more bulk inside, the less water it will take to fill it.  Also, hydro growers who use a media such as rock wool cubes will usually only flood the grow bed to about 1-2" below the tops of the cubes. 

Now for the pump.  You need to know how long your pump will be 'on' while it's flooding your grow bed.  Most growers flood for a 15 minute period (as most simple timers have a minimum 'on' time of the same).  So let's say you will be watering for 15 minutes each time and need to make sure your grow bed will get completely flooded during that time.

Most submersible water pumps are sized by gallons per hour (GPH).  Since we are assuming you will be flooding for 15 minutes and need to fill the grow bed with 34.7 gallons of nutrient within that time, and since we know 15 min is 1/4 of an hour, we multiply 4 x 34.7 to find out what GPH size pump you will need (4 x 37.4= 139 GPH).  So for flooding a 2'x4'x7" flood tray to the top with 34.7 gallons, you would need a submersible water pump that is 139GPH minimum.

Tip: Because hydroponic ebb & flow systems have an overflow drain, you shouldn't have to worry about having a submersible water pump that is a little larger than the minimum.  You want to make sure your plants will get watered.