Short answer. Majority is by share, not by headcount. Under Article 492 of the Civil Code, a resolution is binding only when approved by co-owners who represent the controlling interest — more than half of the total ownership value. Four co-owners each holding a 10% share cannot outvote one person holding 60%, no matter how many of them there are.
What the law says
There shall be no majority unless the resolution is approved by the co-owners who represent the controlling interest in the object of the co-ownership.
Civil Code, Article 492 — How Co-owners Decide on Administration. Read the full provision →
How majority is counted
Article 492 sets the rule for administration decisions in co-ownership: resolutions of the majority are binding, but "there shall be no majority unless the resolution is approved by the co-owners who represent the controlling interest in the object of the co-ownership." Controlling interest means more than half of the ownership value — not more than half of the individuals. In a co-ownership where you hold 55% and two others hold 45% combined, you have the majority even though you are outnumbered two to one. The value of each person's share, not the number of people, is what determines whether a resolution carries.
What the majority can decide
The majority's authority under Article 492 extends to administration and better enjoyment of the property — the day-to-day management of the co-owned thing. This covers decisions about how to use or maintain the property, who leases it, how income is distributed, and similar operational matters. It does not cover acts of ownership such as selling or mortgaging the entire property, which require the consent of all co-owners. The majority can direct the property's management; it cannot dispose of the property over the minority's objection.
When there is no majority or the majority is harmful
Article 492 anticipates deadlock and abuse. If the co-owners cannot reach a majority — for example, because the interests are perfectly split — or if the majority's resolution would be "seriously prejudicial to those interested in the property owned in common," any interested party can go to court. The court can then order whatever measures it deems proper, including the appointment of an administrator to manage the property in place of the disputing co-owners. This prevents paralysis when equal or near-equal shares prevent a decision, and protects minority co-owners from majority abuse.
Mixed ownership: part exclusive, part co-owned
Article 492 also addresses a less common situation: where part of a thing belongs exclusively to one co-owner and the remainder is owned in common. In this case, the voting and majority rules apply only to the portion that is actually held in common. The exclusively owned part is not subject to a co-owner vote at all — the sole owner of that portion decides for it independently. This might arise, for example, where a building sits on land that is partly owned solely by one person and partly held together with others.