Flat Rate vs Calculated Shipping Calculator
See whether a flat-rate box or weight-based calculated shipping keeps more margin, across your store's real mix of light, medium and heavy orders.
Your numbers
Your result · updates as you type
Calculated rates would keep more margin
+$1.60 / order
Across 800 orders a month, switching to calculated rates is worth about $1,278 a month with this order mix.
- Flat-rate margin per order
- -$2.70
- Calculated margin per order (blended)
- -$1.10
- Blended calculated cost
- $8.05
A flat-rate box is a bet: you win on heavy items that would cost more by weight, and lose on light ones that would cost less. If your mix shifts (more light items after a new product launch, say), rerun this before you commit to one option storewide.
Breakdown
- You charge the customer
- $6.95
- Flat-rate box cost
- −$9.65
- Light (50% of orders) at $6.10
- Calculated wins
- Medium (35% of orders) at $8.40
- Calculated wins
- Heavy (15% of orders) at $13.75
- Flat-rate wins
- Monthly margin: flat-rate
- -$2,160
- Monthly margin: calculated
- -$882
Flat rate is a bet on your order mix
A flat-rate box charges the same price no matter what's inside, up to the carrier's weight limit. Calculated (weight- and zone-based) shipping charges more for heavier orders and less for lighter ones. Neither is better in general: it depends on where your orders actually fall.
Compare that blended figure with your flat-rate box cost. If most of your orders are light, calculated rates usually keep more margin. If most are heavy, a flat-rate box usually wins, because you're paying one price regardless of weight.
Setting this up in Shopify or WooCommerce
- 01
Find your real mix
Export a month of orders with their shipped weight, and bucket them into light, medium and heavy. Even a rough split beats guessing.
- 02
Shopify
Add shipping rates under Settings → Shipping and delivery. You can set a flat price per rate, or connect a carrier account for calculated rates by weight and zone.
- 03
WooCommerce
Under WooCommerce → Settings → Shipping, add a Flat rate method for a fixed box price, or a Table Rate / carrier plugin for weight-based calculated shipping in each shipping zone.
Pitfalls
- Don't set it once and forget it. A new product line can shift your weight mix enough to flip the better option.
- Flat rate simplifies pricing but hides the loss. If you charge the customer a flat shipping fee too, a run of heavy orders under a flat-rate box can quietly erase the margin on them.
- Compare against what you charge, not just what you pay. The calculator nets out your shipping charge to the customer, since that's what determines your actual margin per order.
Questions people ask
When does a flat-rate box save money?
When most of your orders are heavier than what a calculated rate would charge for that weight and zone. Flat-rate boxes are built around an average, so heavy items ship for less than their real weight-based cost.
When does calculated shipping save money?
When most of your orders are light. Calculated rates charge close to the real cost, which is usually less than a flat-rate box's fixed price for a light item.
Can I offer both?
Yes. Many stores use calculated rates by default and switch to a flat-rate box only for specific heavy or bulky product categories.
Does this account for what I charge the customer?
Yes. Enter what you charge for shipping, and the calculator nets it against each option's real cost to show your actual margin per order.
Related tools
Want this builtinto your store?
I'm Emmanuel, a Shopify and WooCommerce developer. If this tool pointed at something you'd rather have fixed for you (a faster store, cleaner product data, a migration, a custom feature), send me a note.
Emmanuel Asika
Shopify & WooCommerce developer, Dublin
I build and fix online stores: new builds, migrations, speed, SEO and the custom bits apps can't do.
Last reviewed 30 September 2026.