A S$100 restaurant menu price marked ++ does not end at S$119. The service charge enters first, then GST applies to the service-inclusive amount. The exact ending is S$119.90, and that arithmetic explains why a separate percentage tip is normally unnecessary.
Turn two plus signs into three numbers
The bill is multiplication, not simple addition
GST follows the service charge
IRAS treats service charge as part of the total price of the meal, so 9% GST applies after the typical 10% service charge has been added. Adding 10 and 9 to get 19% misses the extra GST on the service portion.
The service charge is commercial, not compulsory law
Singapore does not require restaurants to add 10%. A venue chooses whether to impose it. Hotels and F&B businesses that do impose a service charge may display GST-exclusive prices, but must prominently state that GST and service charge will be added.
No extra tip is the calm default
Visit Singapore describes tipping as not customary. When a restaurant has already added service, paying the displayed final bill is complete and polite. A small additional amount can thank exceptional personal service, but it is not a hidden second obligation.
Hawker and takeaway prices usually have a simpler ending
Self-service hawker meals do not normally carry the restaurant service-charge structure. GST-registered businesses without the service-charge exception should display GST-inclusive prices. Read the sign at the specific venue rather than carrying the ++ formula into every bowl of noodles.
Keep card conversion out of the finale
After checking service and GST, confirm the terminal still shows SGD. A home-currency button is a separate foreign-exchange offer and has nothing to do with the restaurant's taxes or service. Declining it does not change the SGD bill.