If you overload a washing machine it can cause physical problems, but it will also affect the efficiency of the wash. An overfilled washing machine results in the laundry moving round in one large mass. The clothes will not agitate against each other and the washing machine detergent won't be able to circulate or dissolve properly. The washing will not be washed properly. With some liquid detergents there can also be potential serious consequences – Baby badly burned by washing machine detergent.
Overfilling the drum could potentially even cause the door glass to shatter according to manufacturers although as far as I know none of them warn about this directly to customers. There have been over 90 cases reported to Whitegoodshelp over the last several years – Washing machine exploding door glass danger so make sure you don’t ram the drum full.
However, you do need to fill the drum in order to get the most economical use from your machine and to prevent under-loading, which is also bad . Under-loading the machine causes problems by making the weight of clothes more likely to accumulate on one side of the drum. This can make the load out of balance and cause violent banging on spin. (see below)
How much laundry should I put in the drum when loading a washing machine?
Try to fill the drum for economy, but pat the clothes down lightly and make sure you can feel a good gap between the top of the clothes and the top of the drum. Bear in mind that the drum may look really full, but once water comes in and the drum turns, many items will shrink in bulk. I would try to fill the drum around 3 quarters full so that there is plenty of room for the laundry to move around. The laundry needs space to fall into when the drum revolves. Check your instruction manual to see if there are good instructions for your machine.
Under loading the washing machine
Under loading the washing machine can also cause problems. In the old days it often resulted in violent out of balanced loads causing damage to the washing machine. These days under-loading is more likely to result in the washer simply refusing to spin so you end up with a finished cycle but wet laundry. If you have experienced some loads reaching the end of the cycle and not going into fast spin, then one possible explanation could be if you have under loaded it or are trying to wash and spin just one or two items.
More detailed washing machine loading issues and advice
I have two more articles on this subject, both of which are essential reading to fully understand the issues surrounding loading and balancing a washing machine drum. For a full explanation of the out of balance protection systems used by modern washing machines check out my article – Washing machine won’t spin just one item or very small loads. For a full explanation of the causes of a washing machine thumping around violently on spin read – Washing machine jumps around and bangs on spin