Core Weight and SAP Ratio in Sanitary Pads: RFQ Guide

Two factories quote the same 245 mm day pad. One comes in lower. The samples look alike on a desk, feel similar in the hand, and both arrive with a printed absorbency claim on the pack. Six weeks later the cheaper batch draws complaints about surface wetness. Nothing visible changed. What changed were two numbers nobody wrote down: the core weight and the share of superabsorbent polymer inside that core.

Those two variables decide how much fluid a pad holds, how dry the surface feels after a load, how thick the pad sits in the pack, and where the unit price lands. Neither is printed on the retail pack, and most pro forma invoices list only a length and a total price. For a brand owner, importer or institutional buyer writing a specification, they are the two lines worth arguing about before an order is placed.

This guide explains the mechanism, the trade-off and the values to pin down in a request for quotation (RFQ). It does not hand you a single "correct" gram weight or ratio, because outside a confirmed specification and a third-party test report no such number exists. What it does is let you make two factories quote the same product, so the price difference means something.

Why two invisible numbers decide what you receive

A sanitary pad is a layered structure. From the body side down, the usual build is a topsheet, an acquisition or distribution layer, the absorbent core, a backsheet, and the wings or elastics that hold the pad in place. The core carries almost all of the fluid. Every other layer moves fluid toward it, keeps it inside, or keeps it off clothing.

Inside the core, two materials do different jobs. Fluff pulp is a bulky cellulose fibre mat: it grabs liquid fast and spreads it along the pad. SAP is a polymer that takes up many times its own weight in saline and turns into a gel that holds fluid under pressure. Pulp acquires, SAP retains. A core that is mostly pulp feels soft and takes fluid quickly, but gives some of it back when the wearer sits down or crosses her legs. A core that is mostly SAP stays drier under pressure and can be built thin, but pulls fluid away from the surface more slowly and costs more per gram.

Core weight is how much of that mixture goes into one pad, written either as grams per piece or as grams per square metre (basis weight). SAP ratio is the share of the core, by weight, that is polymer rather than pulp. Move either one and you have a different product, even though the pad looks the same.

Read the table as a set of linked knobs, not as five independent switches. Raising SAP share without touching the acquisition layer is the single most common way to make a pad that measures well on capacity and feels wet in use.

What core weight actually measures

Core weight is a quantity of material, and the unit matters more than buyers expect. Grams per piece is what you feel and what you pay for. Grams per square metre is what a production line controls, because it is the setting on the forming drum. A 245 mm pad and a 290 mm pad made at the same basis weight do not contain the same total grams; the longer pad simply holds more of the same material.

So the first question to ask a factory is which unit it is quoting. If the answer is basis weight, ask for the corresponding grams per piece for each length in your range. If the answer is grams per piece, ask whether that figure covers the core only or the whole pad.

That second question is not pedantry. A finished pad also carries the topsheet, the acquisition layer, the backsheet, the release paper, the wings and the adhesive. A pad can be heavier than another and still hold less fluid, because the extra grams went into film and paper rather than into the core. Two pads with the same total weight can perform differently, and the difference is where the weight sits.

Tolerance is the part most RFQs leave out. A core weight without a plus-or-minus range is an aspiration, not a specification. Ask for the value and the tolerance together, and ask what happens at the edges of that tolerance: does the line stop, or does the material keep running and get sorted later?

What the SAP ratio does, and why there is no universal answer

SAP ratio is usually written as a percentage of core weight. It is a design decision, not a quality grade. A higher ratio is not better in the abstract; it is better for some products and wrong for others.

Here is the mechanical reason. Fluid arriving at the topsheet has to be pulled through the acquisition layer and distributed through the core before the polymer can gel it. Pulp does that transport work through capillary action. SAP does almost none of it. Once SAP has gelled, however, it holds fluid far more tightly than pulp does, and it does not release that fluid when pressure is applied.

Push the ratio up and you get a thinner pad with better retention under pressure and a drier surface after the first load. Push it too far and the first fluid arriving at the surface has nowhere fast to go, the topsheet stays wet longer, and the wearer reports leakage even though a laboratory capacity test looks fine. Push it down and you get a soft, fast, bulky pad that feels dry for the first few minutes and then gives fluid back under pressure.

That is why the honest answer to "what ratio should I use" is: the one that matches your pad's length, your target capacity, your topsheet and acquisition layer, and your price point. A factory that answers with one number across every SKU in your range is not designing; it is guessing.

How the two levers move together

Core weight and SAP ratio do not act independently. Four outcomes come out of the pair, and every one of them is a trade-off against the others.

1. Total absorbency. Held mostly by core weight. More material means more capacity, at a roughly proportional material cost.
2. Rewet, or surface wetness after load. Held mostly by SAP ratio and by how well the acquisition layer is matched to it. This is the number behind most "it leaked" complaints that a capacity test cannot explain.
3. Thickness and hand feel. A higher SAP ratio lets you hit the same capacity in a thinner pad. A higher pulp share gives loft and softness.
4. Unit price. SAP costs more per gram than pulp, and the acquisition layer needed to make a high-ratio core work costs money too. Two pads with identical capacity can have different costs because they reached that capacity by different routes.

A useful way to hold this in your head: core weight buys you capacity, SAP ratio buys you dryness under pressure, and both are paid for in cash and in thickness. If a quotation drops the price while claiming the same capacity and the same thinness, ask which of the two levers moved.

How product positioning moves the balance

The right setting changes with the job the product has to do. This is where a buyer's range plan should drive the specification, rather than the specification driving the range.

• Thin day pads. The buyer wants discretion. That pushes toward a higher SAP share at a moderate core weight, with an acquisition layer good enough to keep the surface dry. Cost per gram is higher; the pack is slimmer.
• Heavier flow and night-use pads. The buyer wants capacity and coverage. That pushes toward higher total core weight and a wider, longer core, with the ratio set to keep rewet acceptable at that load. Night-use products should be described as designed for overnight wear, and any performance language should stay away from absolute leak-proof promises.
• Postpartum and institutional supply. The buyer wants high total capacity and forgiveness of fit. Bulk is acceptable, acquisition speed matters, and unit cost is usually weighed against fewer changes per patient or per resident.
• Disposable menstrual pants. Here the core has to stay narrow and flexible so it moves with the body. The same capacity target that is easy in a flat pad becomes a design problem in a pant, which is why core construction and leg elastic fit have to be specified together. In US and Canadian markets you will hear period underwear; in the UK, Australia and New Zealand period pants; across English-speaking Africa sanitary pants. The unified trade term we use for the disposable category is disposable menstrual pants, and they are not the same product as washable cloth-pad underwear, which is what the reusable terms usually refer to in those markets.

Pants and pads also differ in where the fluid lands, which is why a core that performs well in one format does not transfer to the other without re-testing. See our note on how SAP core design supports disposable menstrual pants programs for the pants-specific view, and the disposable menstrual pants manufacturing program page for the format itself.

Seven values to pin down in your RFQ

An RFQ that says "245 mm, ultra thin, high absorbency" will get you ten different products and ten prices that cannot be compared. The following items turn it into a specification. None of them need a magic number from you; they need a number from the factory, plus a tolerance, plus a test report that confirms it.

Two more lines are worth adding even though they are not numbers. First, a change-control clause: any change to core material, SAP supplier or core weight has to be notified before production, not after. Second, a sample-lock clause: the approved sample is defined by the written specification, not by the physical pad in your folder, because a physical sample cannot be referred to in a dispute two shipments later.

Which test reports to ask for, and what to read in them

A specification is only as good as the evidence behind it. Ask for the following, and read the header before you read the numbers.

• Absorbency test. The standard saline absorption measurement for the pad. Check the method, the sample size and the date.
• Rewet test. How much fluid comes back to the surface under a defined load. This is the number that correlates with comfort complaints, and the one most often missing from a quotation pack.
• Thickness measurement. Under a stated pressure, at a stated position.
• pH result. A routine hygiene and material check for the finished pad.
• Incoming material records. Gram weight and absorption rate for the pulp and the SAP lot used, plus the topsheet and backsheet lots.

On standards: the Chinese national standard for sanitary pads is GB/T 8939, and the version currently in force is the 2018 edition. A 2025 revision has been published but takes effect on 1 January 2027, so a test report issued today should still be read against the 2018 edition unless your market requires something else. If you sell into a market with its own absorbency or labelling rules, name that standard in the RFQ rather than assuming the factory's default matches it.

At factory level rather than product level, ask for the ISO 9001 certificate and the GMP documentation, and check that the company name on the certificate is the same legal entity that will sign your contract and receive your payment. Those documents say the factory has a quality system; they do not tell you what is in your pad. Only the product-level test report does that.

Our sanitary pad quality control and inspection process page covers the inspection side in more detail, and the sanitary pad manufacturing process walkthrough shows where core weight is actually set on the line.

Four mistakes that cost buyers money

1. Comparing unit price without core weight

Two quotations for the same length are not comparable until both state the core weight and the SAP share. A lower price with a lighter core is not a saving; it is a different product sold at the same shelf price.

2. Approving a sample but not a specification

Samples are made carefully. Production is made quickly. If the only definition of your product is the sample in your hand, you have no way to reject a batch that is close but not the same. Write the numbers down, including tolerances.

3. Accepting "same as last time" after a material change

A change of SAP supplier or pulp lot can move rewet without moving absorbency. Ask for a fresh test report whenever the core material changes, even if the gram weight stayed inside tolerance.

4. Specifying maximum absorbency on every SKU

Capacity you do not need is money spent on material, thickness and freight. A range works better when each SKU is specified for the flow level and wearing situation it is actually sold for.

Frequently asked questions

Is a higher SAP ratio always better?
No. A higher ratio improves retention under pressure and allows a thinner pad, but it also makes the acquisition layer work harder. Without a matched topsheet and acquisition layer, a high-ratio core can feel wet on the surface even when its laboratory capacity is high. The ratio should be set together with the acquisition layer, not on its own.

What is the difference between core weight and pad weight?
Core weight is the mass of the absorbent material only. Pad weight includes the topsheet, acquisition layer, backsheet, release paper, wings and adhesive. A heavier pad can hold less fluid than a lighter one if the extra grams are in film and paper rather than in the core. Ask which figure you are being given.

Should I ask for grams per piece or grams per square metre?
Ask for both. Basis weight is what the forming line controls; grams per piece is what you pay for and what the wearer feels. Asking for both also makes it obvious when a longer pad quietly keeps the same basis weight and therefore contains more material than you budgeted for.

How do I stop a factory from quietly lightening the core?
Three things together: write core weight and SAP share into the specification with tolerances, require a pre-shipment test report against those values, and add a change-control clause so any material or setting change has to be notified before production. Without the third item, the first two only tell you after the fact.