It can be pretty obvious when someone is overwhelmed, upset, and acting out. It can be even more apparent when your partner is the one experiencing the distress.

You may notice that his voice is louder or he's speaking more quickly.
You may see that she has begun "the silent treatment".
He may begin acting impulsively or irrationally.
She may be more argumentative and bringing up past issues.

When you notice these reactions and feel your impulse to respond with "calm down", ask yourself why you need to experience his "calm" yourself, and consider one or more of these options instead:

Listen. The power of listening can be invaluable. Giving your partner freedom to speak removes his assumption that you may feel he is "overreacting" and promotes a safe place to be vulnerable. You can invite your partner into the safe exchange, saying "I see this may be hard for you, tell me more..."
Notice. What has been happening in and around your partner? What has she experienced at work? What is the atmosphere like at home? How has she been sleeping? Has she felt ill recently? Do things seem fragile financially? Are her other relationships strong? Observing what is happening in and around your partner helps to better understand the context of her experience.
Ask questions. Out of all the information and facts you know, there is likely a mountain of knowledge you have yet to learn. Find ways to ask your partner questions that let him hear your interest in the details of his experience. Open-ended questions can be helpful and create a dialogue that is full of curiosity and respect. You know him well enough to know he is upset. Ask him what led him there. See how it grew from the onset of the discomfort to the extent of the state of overwhelm.
B R E A T H E. Allow yourself to take a moment to re-group, slow down, and invite your partner to experience this with you. Model the slow inhale - pause - and exhale even more slowly. Finding this rhythm together ignites unity and communicates the desire to overcome this together, no matter the extent of the battle.
Know your strengths independently and as a couple. Many couples jump straight to "solving the problem", which often communicates to the partner in distress that she is incapable of finding a solution. However, after you've walked through the previous steps, you've already laid the foundation. This foundation can then be the catalyst for either feeling comfortable enough in the distress to tolerate it more effectively OR gaining the courage to overcome, change, or resist it. When you know your own strengths, you can better support your partner's decision, and when you know your partner's strengths, you can reinforce his bravery by affirming these stalwart attributes.

When "CALM DOWN" are the next two words to come out of your mouth, consider this: you can either calm the fire or enhance the damage. Crisis can be averted & solutions can be considered simply by looking within, evaluating your need to have your partner "CALM DOWN", and then invite your partner to the safe place for validating his feelings, observing and noticing the context of what is happening, asking questions about his experience, sharing a unified calming breath, or declaring each other's strengths. One or all of these suggestions in tandem can elicit a feeling of camaraderie and support - a natural recipe for success in any relationship.