How to Calculate Hay Tarp Size: Bale Stack Cover Formula

Updated: May 27, 2026

The Hay Tarp Formula

A hay tarp must cover the top of the stack and drape down all four sides, with extra tarp on the ground to anchor it under the bottom bale row:

Tarp length = stack length + (stack height × 2) + (tuck-under × 2)
Tarp width  = stack width  + (stack height × 2) + (tuck-under × 2)

Tuck-under: The extra tarp that goes under the bottom row of bales on each side. This is the primary anchor method. Standard: 2–3 ft per side.

Step 1 — Measure Your Hay Stack

Measure the outside dimensions of the full stacked bale pile:

Length: Longest side, from outer edge of first bale row to outer edge of last
Width: Shorter side
Height: From ground to top of highest bale

Example stack: 30 ft long × 20 ft wide × 10 ft high (typical 4-row square bale stack)

Step 2 — Calculate Tarp Size

Using the example with 2 ft tuck-under per side:

Tarp length = 30 + (10 × 2) + (2 × 2) = 30 + 20 + 4 = 54 ft
Tarp width  = 20 + (10 × 2) + (2 × 2) = 20 + 20 + 4 = 44 ft

You need a 44 × 54 ft tarp. No standard tarp covers this exactly. Options:

Two 25 × 40 ft tarps overlapped 6–8 ft at the ridge
One custom 50 × 60 ft tarp (ordered from an agricultural supplier)

Step 3 — Account for Stack Shape

Flat-top stack (square bales stacked in rows): Use the formula above directly.

Peaked-top stack (square bales stacked in a pyramid/barn shape): Use the total height including the peak. The tarp still runs from ground on one side, over the peak, to ground on the other side.

Round bales in a row (end-to-end):

Length = total row length (diameter × number of bales)
Width = single bale diameter
Height = single bale diameter (they’re round — height = diameter)

Round bale stack (stacked 2 high):

Height = 2 × bale diameter (usually 5 × 2 = 10 ft for 5 ft diameter bales)

Common Hay Stack Tarp Sizes

For square bale stacks:

Stack Size	Height	Tarp Needed	Nearest Standard
20 × 10 ft	8 ft	28 × 36 ft	30 × 40 ft
30 × 20 ft	8 ft	44 × 36 ft	None — use two 25×40
30 × 20 ft	10 ft	44 × 54 ft	None — use two 25×40
40 × 20 ft	10 ft	44 × 64 ft	Two 25 × 40 tarps (overlap)
50 × 30 ft	12 ft	58 × 74 ft	Custom order

Anchoring Methods

Tuck-under (recommended): Tuck 2–3 ft of tarp under the bottom bale row on all sides. The weight of the bales holds the tarp in place. This is the most secure method without stakes.

Bungee cords: Run across the tarp from grommet to grommet through a bungee net. Combine with tuck-under for maximum security.

Sand bags or tires: Place along the base edges if tuck-under is not possible (e.g., bales on pallets).

Tarp clips: For tarps without grommets. Clips grip the tarp edge and attach a rope or bungee. For mil ratings, UV specs, and what to look for when buying a hay tarp, see the Hay Tarp Buying Guide.

Silage Tarps

Silage tarps (for covering corn silage, haylage, and fermented forage) follow the same formula but with larger tuck-under (3–4 ft per side) because air exclusion is critical. For full silage tarp sizing, weighting, and edge-sealing requirements, see the Silage Tarp Guide:

Tarp length = pit/bunker length + (pit wall height × 2) + (tuck-under × 2)
Tarp width  = pit/bunker width + (tuck-under × 2)

Silage tarps are typically 4–6 mil UV-stabilized polyethylene film. They are single-season use and must be weighted continuously (tires every 3–4 ft) to exclude air.

Use the Tarp Size Calculator — select Hay / Silage Stack for instant calculation.