Matched stiffness is a cube root
Bending stiffness of a solid panel goes as the cube of its thickness, so working backwards from a stiffness to a thickness is a cube root. That single fact is why cored panels look so good on paper and why the saving is so hard to guess. Take the default panel: two tenth-inch skins either side of half an inch of core has the same bending stiffness as a solid laminate 0.601 inches thick. The sandwich is 0.7 inches overall and weighs 2.375 pounds per square foot including the bonding resin on both faces. The solid weighs 5.015. That is a saving of 2.64 pounds per square foot, or 52.6 percent.
Now double the core to one inch. The matched solid goes to 0.900 inches, not to 1.204 — up by 49 percent for a core that doubled. The sandwich weight goes from 2.375 to 2.583. The saving widens, but nothing about it is proportional, and anyone estimating by scaling the previous answer will be wrong in a direction that depends on where they started.
The two ends where the sandwich loses
At very thin cores the sandwich carries weight the solid does not: the resin or adhesive that bonds each skin to the core, on two faces. With four ounces per square foot per face that is half a pound per square foot of pure penalty, and it is there whether the core is an eighth of an inch or two inches. Put a sliver of core in a panel and you end up heavier than the solid laminate you were trying to beat: on the default skins and core density the two only break even at 0.065 inches of core, and anything thinner than that loses.
At the other end the arithmetic turns again. Core weight grows in a straight line with thickness. The matched solid thickness grows as the cube root of the second moment, which for a thick core is roughly the two-thirds power of the core thickness. A straight line always overtakes a two-thirds power eventually. With a five pound foam and tenth-inch skins that crossing sits thousands of inches out and is a mathematical curiosity. With a dense core — heavy end-grain balsa, a plywood core, a thick honeycomb quoted at a high density — and thin skins, it can land inside a range somebody might actually build in. The page scans for it rather than assuming either way.
Skin balance, and why even usually wins
Move an eighth of an inch of skin from the inner face to the outer and the panel weighs exactly the same. It does not bend the same. Bending stiffness depends on how far material sits from the neutral axis, and making one skin thick pulls the neutral axis towards it, shortening the lever arm on both faces at once. For two skins of the same laminate the even split gives the most stiffness for a given weight of skin, and the page shows your split against the even one so you can see what an unbalanced layup costs.
There are perfectly good reasons to build unbalanced. The outer skin takes impact and abrasion and gets faired and sanded; the inner one may only need to hold the core. This page does not argue against that, it just prices it in stiffness so the trade is visible.
What equal stiffness does not mean
It is worth saying plainly, because the comparison invites the wrong conclusion: two panels with equal bending stiffness are not interchangeable. The sandwich has failure modes the solid does not have at all. The core can shear before either skin does anything. A thin skin over a soft core can wrinkle or dimple under a local load. The bond line is a separate thing that can fail on its own. A bolt through a cored panel crushes the core unless something is done about it, and a screw hole that lets water in can wet a core for feet in every direction over a season.
None of that appears in a second moment of area, so none of it appears here. This page answers exactly one question — what would a solid laminate of the same bending stiffness weigh — and everything else about the choice belongs with the person engineering the part.
Questions people ask
How much lighter is a cored panel than a solid laminate?
For the panel in the form — two 0.1 inch skins, half an inch of 5 lb core, 100 lb per cubic foot laminate and 4 oz per square foot of bonding resin on each face — the sandwich weighs 2.375 lb per square foot and a solid laminate of the same bending stiffness would be 0.602 inches thick and weigh 5.015. That is 2.64 lb per square foot saved, 52.6 percent. Change the core density or the skin thickness and it moves a long way, which is why the page computes it rather than quoting it.
Can a cored panel ever be heavier than a solid one?
Yes, at both ends. With a very thin core the bonding resin on two faces is weight the solid laminate never carries, so the sandwich can lose before it starts. And because core weight grows linearly with thickness while the matched solid thickness only grows as roughly the two-thirds power, a dense core with thin skins eventually loses again at the thick end. The page scans core thicknesses from a thirty-second of an inch up to 24 inches and reports every crossover it finds instead of assuming there is not one. On the default numbers it finds one at 0.065 inches, below which the bonding resin costs more than the core buys.
Why is the matched solid thickness not proportional to the core?
Because bending stiffness goes as the cube of thickness for a solid section, so the thickness for a given stiffness is a cube root. Doubling the core in the default panel takes the matched solid from 0.602 to 0.900 inches, up 49 percent rather than 100. Every intuition built on proportional scaling gives the wrong answer here, usually in the direction of overestimating what more core buys.
Should both skins be the same thickness?
For stiffness per pound of skin, yes — an even split puts both faces as far from the neutral axis as they can get, and making one thick pulls the axis towards it and shortens the lever on both. The page shows your split against an even one of the same total skin weight so the cost is visible. There are good reasons to build unbalanced anyway, since the outer skin takes the impact and the sanding, and this page prices that choice rather than arguing with it.
Does this tell me if my panel is strong enough?
No, and it is not able to. It compares the bending stiffness of two panels and tells you what each weighs. Whether either is adequate depends on the loads, the span, the edge conditions, the service and a set of failure modes that do not appear in a second moment of area at all — core shear, skin wrinkling, bond line failure, core crush at fasteners, water ingress. That is engineering, and it needs an engineer.
Should I count the core bending stiffness?
Only if you have both moduli from real data, which is why the field is off by default and asks for a ratio rather than two numbers. For a soft foam against a glass laminate the ratio is small and the core contributes very little bending stiffness on its own, so ignoring it is the conservative direction. For a stiff core the contribution is not negligible and leaving it out understates the sandwich. Either way the figure has to come from your core datasheet and your own laminate, not from this page.