Sell by Weight & Length for OpenCart

Overview

Sell cheese by the kilogram, fabric by the meter, flooring by the square meter and paint by the liter. Shoppers enter decimal quantities (1.25 kg, 2.5 m), pick preset amounts and see the line total while they type. The cart, the checkout, the order, the invoice and the stock all use the exact quantity.

• 15 sale units: kg, g, lb, oz · m, cm, ft, yd, in · m², ft² · L, ml, gal, fl oz
• Quantity step, minimum, maximum, default quantity and up to 8 preset buttons per product
• Exact line totals: 1.250 kg × $12.99 = $16.24, rounded once
• Exact decimal stock (8.750 kg); the core Quantity field shows it rounded up
• Quantity discounts in the sale unit; shipping weight × quantity; taxes, coupons and payment gateways work unchanged
• No core file changes and no OCMOD patches: OpenCart events only
• OpenCart 4.0.2.0 – 4.1.x and 3.0.3.x – 3.0.5.x, one license for both

Installation

Requirements

OpenCart 4

1. Extensions > Installer: upload kv_fractional_quantities.ocmod.zip, then click Install next to it in the list of uploaded packages.
2. Extensions > Extensions, choose the type Modules: find Sell by Weight & Length and click Install (green +).
3. Click Edit (blue pencil), switch Status on, enter your license key and click Save.

OpenCart 3

1. Extensions > Installer: upload kv_fractional_quantities-oc3.ocmod.zip.
2. Extensions > Modifications: click Refresh (top right).
3. Extensions > Extensions, choose the type Modules: find Sell by Weight & Length and click Install (green +).
4. Click Edit, set Status to Enabled, enter your license key and click Save.

After installing

Open a product (Catalog > Products > Edit) and choose its sale unit in the new Sell by measure tab. See Configuration.

Updating

Upload the new package the same way. Your settings and product units are kept. On OpenCart 3, click Refresh in Extensions > Modifications afterwards.

Uninstalling

Extensions > Extensions > Modules > Sell by Weight & Length > Uninstall removes the events, the settings and the extension tables (product units, exact stock, measured order lines). Products keep the rounded-up stock in their Quantity field and are sold in whole units again. Existing orders keep their lines and the "Quantity: 1.250 kg" detail.

Configuration

Module settings

Extensions > Extensions > Modules > Sell by Weight & Length > Edit

Product settings

Open a product (Catalog > Products > Edit), tab Sell by measure, and click Save.

The core Minimum Quantity field (Data tab) is not used for products sold by a measure; use the fields of this tab.

Stock

OpenCart stores whole quantities only, so the extension keeps the exact stock next to the core field:

• Quantity (Data tab) shows the exact stock rounded up (8.750 kg → 9), so the product stays in stock while anything is left.
• Exact stock is the real quantity. Orders take exactly what was ordered; canceled orders give it back.
• If you, an import or another extension change Quantity to another whole number, that number becomes the exact stock (10 → 10.000 kg). To set a fraction, use Exact stock.
• With Display Stock on (System > Settings > Option), the product page shows "8.750 kg".

Quantity discounts and specials

Quantity discounts (Discount tab) work in the sale unit: a discount from 5 applies from 5 kg. The price is chosen by the whole units of the quantity (5.5 kg uses the "5 or more" price). Specials apply as usual.

Shipping by weight

Set the product Weight to the weight of one sale unit (1 kg for a product sold per kg). The shipping weight of a line is weight × quantity.

License

The License section of the module settings shows the status, the licensed domain and Check now.

What switched off means: without a valid license (no key after the 14-day start period, an invalid or revoked key, or a domain the license does not cover), products are sold in whole units again: the decimal quantity field and the preset buttons are hidden and no new decimal quantities are accepted. Carts and orders that already hold a decimal quantity keep their exact quantity, price and stock handling. The admin panel and your settings always stay available.

Usage

Product page

The quantity field takes decimals with a dot or a comma (1.25 or 1,25). The unit is shown next to the field, the preset buttons fill in a quantity, the arrow keys move by the step, and the line total is shown while the shopper types. Quantities off the step, below the minimum or above the maximum are corrected.

On product lists, Add to Cart adds the default quantity of the product (the minimum when no default is set).

Cart and checkout

• Each product sold by a measure is one cart line. Adding the same product again adds to that line (1.25 kg + 0.5 kg = 1.75 kg).
• The quantity field of the cart takes decimals; the unit price column shows "$12.99 / kg".
• The mini cart shows "x 1.250 kg".
• If a quantity is corrected (step, minimum, maximum), the success message says so: "The quantity of Aged Cheddar was adjusted to 1.235 kg."

Prices, taxes and totals

The line price is the unit price (after option prices, quantity discounts and specials) × the quantity, rounded to the cent once: 12.99 × 1.25 = 16.2375 → $16.24. Everything after that uses the line price, exactly as for any other product: taxes, coupons, reward points, totals, shipping by weight.

Orders, e-mails and invoices

An order line of a measured product has quantity 1 and the exact line price. Two details under the product name keep the measure wherever OpenCart shows product options: the checkout, order e-mails, the customer's order page, the admin order page, invoices and shipping lists.

• Quantity: 1.250 kg
• Price per kg: $12.99

Order status and stock

When an order enters a processing or complete status, the exact quantity is taken from the stock (10 → 8.750 kg); when it is canceled or moves back to another status, it is given back. This follows your Processing Order Status and Complete Order Status settings, like the core stock handling.

Payments

Payment gateways receive each measured line as "1 × line total" (Aged Cheddar, 1 × $16.24). Gateways that only accept whole quantities therefore work unchanged.

FAQ

Why does the admin order page show quantity 1?

OpenCart stores whole quantities only. A product sold by a measure is one order line ("1") with the exact line total; "Quantity: 1.250 kg" and "Price per kg: $12.99" under the product name keep the measure.

Are prices exact to the cent?

Yes. The line price is the unit price × the quantity, rounded once: 1.250 kg × $12.99 = $16.24. Taxes, discounts and the order total are computed from that line.

Will PayPal or Stripe accept the order?

Yes. They receive quantity 1 with the line total as its price, so the sum of the lines always equals the order total.

Do I need to edit my theme?

No. The extension uses OpenCart events and the standard quantity field of the product page (name="quantity"). It works with the default themes of OpenCart 4 and 3 and with themes that keep that field.

Does it change core files or use OCMOD?

No. It changes no core file and ships no OCMOD patch.

Does it work with product options?

Yes. Option prices are added to the unit price and option weights to the unit weight. Option stock (Subtract Stock on an option value) counts order lines, not kilograms.

What do sales reports count?

Reports that count products count order lines: 1.250 kg is one product. Amounts (revenue, taxes) are exact.

What happens to the stock when I change Quantity by hand?

The number you enter becomes the exact stock (12 → 12.000 kg). To set a fraction, use Exact stock in the Sell by measure tab.

Can I edit a measured line in the admin order editor?

Not in this version: a measured product added in the admin order editor is added in whole units of its sale unit. Status changes, cancellations and invoices work as usual.

Are subscriptions supported?

No. A product bought with a subscription plan (recurring profile on OpenCart 3) is sold in whole units.

What happens if the license lapses?

Products are sold in whole units again; existing carts and orders keep their exact quantities, prices and stock handling. See License.

Does the license check slow down my storefront?

No. The check runs in the admin panel, once a day. The storefront never contacts the license server.