Calculator
Example Data Table
| Finished cover size | Poly fill (medium) | Feather/Down (firm) | Notes |
|---|---|---|---|
| 16 × 16 | 18 × 18 | 19 × 19 | Great for small patio chairs. |
| 18 × 18 | 20 × 20 | 21 × 21 | Most common garden accent size. |
| 20 × 20 | 22 × 22 | 23 × 23 | Looks best with dense inserts. |
| 12 × 20 | 14 × 22 | 15 × 23 | Lumbar shape; add height for loft. |
| Round 16 diameter | 18 diameter | 19 diameter | Round covers hide sizing errors well. |
Examples assume non-stretch fabric and typical seam construction.
Formula Used
The calculator first finds the finished cover size. If you enter cut fabric size, it subtracts seam allowance on both sides:
- Finished width = Cut width − 2 × Seam allowance
- Finished height = Cut height − 2 × Seam allowance
- Finished diameter = Cut diameter − 2 × Seam allowance
It then applies an oversize value to recommend the insert:
- Insert size = Finished size + Oversize
- Oversize changes with insert type, firmness, and fabric stretch.
How to Use This Calculator
- Choose the pillow shape that matches your garden cover.
- Select finished size or cut fabric size as your input.
- If using cut size, enter the seam allowance you’ll sew.
- Pick insert type, firmness, and fabric stretch level.
- Press Submit to see the recommended insert above.
- Download CSV or PDF to save your sizing notes.
Pillow Insert Sizing Guide for Garden Cushions
1) Why insert size matters outdoors
Outdoor cushions face sun, moisture, and frequent use, so they flatten faster than indoor pillows. A correctly sized insert helps the cover keep its shape and reduces corner collapse. As a practical rule, many decorators aim for a “full face” look by sizing the insert slightly larger than the finished cover. For patio seating, a 5–10% loft increase is common. In inches that often means 1–3 in; in metric, about 2.5–7.5 cm. Oversizing beyond 3 in can strain zippers and seams.
2) Typical garden pillow dimensions
Common patio sizes include 16×16, 18×18, and 20×20. Lumbar styles often appear around 12×20 or 14×24, while round accents are frequently 14–18 in diameter. This calculator supports square, rectangle, and round shapes so you can match most garden seating layouts.
3) How oversize is applied
The calculator converts your input into a finished cover size, then adds an oversize allowance. Oversize is kept within 0–3 inches (or the equivalent in centimeters) to stay realistic for retail inserts. Poly fill usually needs less oversize than feather/down, while high-stretch covers often need less oversize to prevent distortion.
4) Seam allowance and finished size
If you enter cut fabric size, the calculator subtracts seam allowance on both sides. For example, a 19×19 cut panel with a 0.5 in seam allowance becomes an 18×18 finished cover. That finished size is the correct basis for selecting insert dimensions, especially for crisp garden cushion covers.
5) Recommended sizing ranges
For a tailored outdoor look, a common target is +1 to +2 inches oversize for poly fill, and +2 to +3 inches for feather/down when the fabric has little stretch. Foam inserts are typically closest to finished size for easy insertion and smooth edges. When shopping, compare the label size to your finished cover, not the cut fabric. If your result sits between standard retail sizes, choosing the next size up often looks better for poly or down, while foam should stay closer.
FAQs
1) Should the insert be bigger than the cover?
Usually yes. Many garden pillows look best with +1 to +3 inches oversize, depending on fill type, firmness, and fabric stretch.
2) What if I only know the cut fabric size?
Select “Cut fabric size” and enter seam allowance. The calculator subtracts two seam allowances to estimate the finished cover before recommending an insert.
3) How do I size a square pillow?
Choose Square and enter one side length. The calculator uses that value for both width and height, then applies the oversize recommendation.
4) How do I size a round pillow insert?
Choose Round and enter the cover diameter. The recommended insert diameter equals the finished diameter plus the calculated oversize, rounded to your chosen step.
5) Does fabric stretch change insert size?
Yes. Stretchy fabrics can accommodate a larger insert without wrinkling, so the calculator reduces oversize as stretch increases to keep the face smooth.
6) What oversize is best for feather/down?
Feather/down often benefits from +2 to +3 inches for a plush garden look, especially with low-stretch covers and a firm appearance goal.
7) Why does foam recommend less oversize?
Foam is less compressible and can be hard to insert when oversized. Matching foam closer to the finished cover helps avoid stress on seams and keeps edges even.