You know the feeling. The cleat bag is riding shotgun, the soccer ball is rolling under the brake pedal, and somebody needs the folding chair for the sideline. The trunk isn't full. It just isn't the trunk you planned for. This is a sizing question, and you can answer it on an ordinary afternoon before the rush starts.
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Measure the floor, not the box
Open the back and check the usable floor with the rear seats up. Then check it again with them folded, because that's the number you'll use on the days you actually fold them. Write down the length, the width, and the height up to the ceiling light. The height is the one people forget. A shelf or a strap can steal a foot of space you were counting on.
Count what rides every afternoon
Walk through one normal week and list everything that goes in the back. One kid's sports bag. The other kid's art folder or instrument case. A cooler for snacks. The chair. The bag of wet things from practice. Write each item on a sticky note and stack them on the hood. That stack will tell you more than your memory does.
Give every bag a home
Pick one bag for each kid and one bin for the shared gear, then decide which ones stay in the car all week and which ones come home. Shared gear like the ball bag and the cone set belongs in one place. Personal gear rides with its kid. When a bag has no fixed spot, it ends up across the back seat, and that's where the afternoon starts to slide.
Plan the fold
Decide which seat folds on which day. If the second row only folds for one of the two kids, say so out loud at pickup so nobody reaches for a seat that's already holding a chair. Keep the heaviest bin low and against the seatbacks, and put a strap or net across anything that could slide forward on a sudden stop. Use whatever tie-down your car came with, and use it every time.
Check before you add anything
Before you add a rack, a cargo box, or a second bin, check three things. Does the extra gear fit once the seats fold? Can it be stacked without blocking the rearview mirror? Will the cargo box be worth hauling on the days it sits empty? Then ask the coach or the program coordinator what they expect the kids to bring, so your count matches theirs instead of your guess.
Keep the count honest
Redo the sticky-note stack at the start of each season. Gear grows. A new sport, a bigger bat, or a duffel that replaces a backpack can turn the trunk you measured last spring into a different trunk. Give it a quick second look every time the gear changes, and the car will keep up with you.